it's probably possible, but it will make your forests spookier.
Ultimately, by how your leafmaws behave and act and what they're made of, I'd say that the biggest requirement you need for a leafmaw predator is simply an omnivorous digestive track. Being part animal, part plant, your leafmaws are essentially a large, land version of an octopus posing as a rock or part of a coral reef, except that some variants have paralyzing tentacles and they can spread through seeds and by breaking off parts of themselves.
This fact means, that, at the end of the day, your average caterpillar might not be a good pick. Not only are they adapted to an herbivorous lifestyle, meaning they could have trouble in eating the entire creature (which is necessary here, because if I got it right these things can reproduce like starfish and the last thing we want is something that just turns them into a bunch of pieces that ultimately end in new organisms and doubles their population), they're not used to or very competent at dealing with their food fighting back in ways other than foul chemicals and poison. If anything, you'd probably be better off looking at carnivorous caterpillars, since it's usually easier to adapt a carnivore to include plant matter than it is to adapt a sluggish herbivore into a competent omnivore hunter.
The problem with carnivorous caterpillars however is that they're ambush predators that normally disguise as parts of leaves and trees, which wouldn't be too bad if leafmaws, unless I got your description wrong, actually relied more on the environment itself. However, they often stand alone, not being necessarily close to any trees or walls the caterpillar could be perched on, meaning that they'd often need to actually go after the leafmaw, thus requiring the less-than-athletic caterpillar to act like a pursuit hunter.
Ultimately, since they come from hydra and octopuses, it's good to take a look at what eats those creatures and see what traits we definitely want:
1-venom resistance. Much like how leatherback turtles can effortlessly eat jellyfish and ratels can eat venomous snakes, your predator will need a natural counter to the venomous variant of the leafmaw. Be it completely immunity or the ability to survive after passing out, your predator needs to be able to deal with the venom.
2-strength/size. A key trait seen in most octopus predators, they're often at least a little larger than them and strong enough to be able to win against them in a wrestle. Morays, dolphins, birds, sharks, Bobbitt worms. All are often bigger (or at least longer) than the octoous and most are at least somewhat stronger in comparison.
With these 2 main traits in mind, it would be fairly easy to make use of larger bears as predators to these creatures, relying on their sense of smell to identify the leafmaw. Many bears, including the mighty grizzly bear, are omnivorous and have a good sense of smell. By relying on them, on larger extinct species or on dire variants you'd only need to add in some venom resistance to make them effective predators for leafmaws (the fact the initial defense mechanism of every leafmaw is its camouflage could make them fairly easy prey, especially if they will not or even cannot easily escape quickly).
but giant insects aren't off limits here and your woods can use some extra nightmare fuel, so let's try to make the caterpillars work
the only way I'd see a caterpillar efficiently preying on leafmaws would be to take an old friend as the base, changing it to make them able to digest plant matter like they once could, adding in the venom resistance (or even allowing them to reuse it, in case at least some leafmaws aren't resistant to the paralyzing venom of the hydra variant) and increase it in size exponentially. By exponentially, I mean making them go from mimicking a stick/twig in a tree to mimicking an actual tree trunk. With this and by giving the square cube law the cold shoulder (sorry mate), we could make something that can fit the bill well enough.
Much like the Bobbitt worm and it's ancestors, these giant predatory caterpillars would stay still, with the main difference that they'd bury their lower end into the ground to better mimic the roots and trunk of a tree and give themselves a better grip on the soil they'd need to remain perched on. Once something came close enough and triggered their long, but thin sensory hairs exposed near the ground, it'd lunge at it with surprising speed, immediately hoisting it up and beginning to dig in. The venom resistance would ensure the caterpillar could eat unimpeded and the size difference would ensure the leafmaws couldn't brute force their way out, as the 6 spiked muscular legs would make sure to keep them in place as they are eaten, most likely alive, by the thing.
To better elaborate how it'd normally work in a documentary-esque description: Your average leafmaw would be at a risk every time they decided to move to somewhere with higher chance of catching food, because all it takes is for them to get too close to the wrong "tree". Should they get within the caterpillar's attack range (which naturally vary depending on the thing's size), it would almost inevitably trigger the sensory hairs by simply walking over them (remember, this caterpillar's legs and a portion of its rear remain buried with its hairs barely sticking out, and thus something with enough weight stepping over these portions of the creature would be enough to signal that something is there), which in turn would immediately let the caterpillar know where the leafmaw (or whatever is causing the disturbance) is relative to themselves. This would allow them to quickly turn their long body around and stab the leafmaw with its spiked legs while rearing them up back into its straightened resting position, already digging in on the newly caught meal. The caterpillars' sheer size difference and venom resistance would mean they'd be able to resist nearly all of the leafmaw's attempts at escaping. Note that, unlike an adapted bear that would actively hunt the leafmaw and much like the leafmaws themselves, these hunting caterpillars are much more passive, being themselves ambush predators, but simply occupying a higher level in the food chain. As a result of this however, these caterpillars would likely be more at home in more densely packed woods where they can easily blend in.
Now: the pros and cons of such a creature existing:
Pros:
these creatures are already known to be fairly efficient and pretty successful predators, and their lack of reliance on sight means they wouldn't be fooled by the leafmaw's camouflage.
much like their real counterparts, these caterpillars can also prey on other creatures, meaning they could keep other monster populations in check.
if their strength also increases linearly with size like in the gigantos, their adult form might be able to serve as a decent form of transportation through the air
Cons:
being ambush predators, these caterpillars would rely on their prey getting close enough, meaning that they won't be ideal if what you want is something that actually chases the leafmaws
being an insect that's gigantic in size, they'll need either severe modifications to their anatomy or a fair bit of magic to work.
Being a caterpillar means these things are infants. They'll need very large hideouts in order to safely begin their transformation into adults. Your world will need either giant trees or safe caves for them to cocoon themselves. You'll also need to worry about how you'll feed the species of mini mothras that will exist in Alendyas (colossal trees and giant flowers might be needed).
Their ability to prey on essentially anything big enough means your humans would now have yet another reason to never let their children play in the woods. To better illustrate my point I made use of my less than stellar drawing abilities and the help of my new original character little Timmy:
So summing up: can caterpillars work as leafmaw predators? If your giant insects are just as capable as their tiny counterparts, then probably yes, we can make caterpillars work so long as we rely Firstly on their carnivorous brethren and make the necessary adjustments in terms of venom resistance as well as their overall size and strength, although they might face problems in later stages of their lifecycle.
