How did this thing get on land?
First question, very important. I assume from the OP that its method of locomotion will not work on land, but it got out of land anyway, so....
If these things are ending up on land despite not currently being equipped to move on land, then natural law dictates that they will adapt or die, same as any other lifeform. The thing is, natural selection and evolution will always work with what's already there, so the real question is:
is whatever method of locomotion they used before something that could work on land?
If they moved using electromagnetic fields to propel the water around them (like a similar question's answer proposed), then they could, quite possibly, develop magnetic levitation and move just fine on land. Otherwise, then things get a little nutty....
Just like barnacles, these things are rock-like and immobile. Barnacles attach to marine organisms, and so can these guys (given a sufficiently large host). Of course, given it lacks mouthparts for filter-feeding, chances are it'd feed like a lamprey off said host, so this would result in big bloodsucking barnacles (BBBs).
2. My precious...
Insects have chitin in a variety of colors, so why not these guys? Given the OP did not mention an excretion method, it may be that they store waste products, like ammonia and such, inside themselves in such a way as to render them toxic, which could naturally lead to A) bright coloration to scare off predators and B) humans who collect these "shinies".
3. Carriage through Camouflage
Oh, if only these things weren't so dang big....anyway, a large enough creature could:
A) collect these things in a pouch and spit them as projectiles,
B) swallow these things to aid in digestion (which, granted, would greatly complicate survival)
C) stick these things on its body for additional protection, or else carry them on their back as a sort of shield that they can hide underneath.
3. Do they need to move?
This is a scenario, which may or may not work. We've all seen a mermaid perched on a rock, yes? Seems typical for them to bask on rocks, after all, predators can't really reach them up there-unless, of course, the rock is the predator!
Naturally, one would assume the mermaids would avoid the man-eating rocks, but what if they don't? What if the mermaids realize the rocks only eat one mermaid every so often and members of the school start sacrificing themselves so another schoolmate can bask on it?
Or what if the mermaids move the rocks themselves to another school's territory, so that they get eaten by the rocks instead, starting an exchange? Or what if curious mermaids regularly go inside the rocks intentionally, arousing the curiosity of their schoolmates and leading them to form a mythos that requires one to be eaten to enter the afterlife?
Otherwise, I'm afraid you've created an almost impossible scenario we can't answer except with additional information about how these things moved before they went onto land, and you may just have to handwave or conceal their mode of transportation beneath their massive bulk.