I want a creature capable of going from a solid structure capable of walking to a liquid or slimy substance clear as water or ocean blue.
How can this be accomplished?
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Octopi are highly flexible, satisfying the constraint of "fitting into any shape". One could easily imagine an octopus-like alien that walks in a more land-animal-like way. Perhaps this began as a way to travel from tidepool to tidepool in search of prey, with speed being necessary to avoid bird-like predators. Octopi already have flexible bodies and strong camouflage abilities, so one could imagine them developing the ability to ripple their skin and change its color in a water-like way, seeking protection from other predators too fast to avoid.
For something a little more exotic, we can consider the sea jelly / jellyfish. Sea jellies can be almost transparent and are already close to liquid in their consistency. One can plausibly imagine the transparency being even greater. Thus, the challenge here isn't with being water-like, but with being able to walk.
The first problem that comes to mind is a lack of bones. While we could use a muscular hydrostat like an octopus arm, the more muscles we add the harder the time we'll have keeping the creature liquid-like. Instead, let's consider something more like a hydrostatic skeleton: we can imagine long sacks that are extremely flexible when empty (think a plastic bag in the ocean) but become stiff when filled with water or air (think a party balloon). Using these sacks, the creature can form an internal semi-rigid structure when needed. We could imagine the creature using this to walk, either with muscles or by further inflation and deflation of these sacks, deflating a leg to take the weight off it, then inflating a sack behind it (in the "armpit") to push it forward, then inflating the leg again. The creature would still need some muscles to inflate the sacks, but this could be a single "lung" with plumbing throughout the body. When in liquid mode, the lung could be kept at the center.
The other big problem would be energy input. Sea jellies, as a rule, drift with the ocean currents, expending minimal energy and eating passively. You would need a mechanism for them to get all the energy needed for this walking. Hunting is a no-go: normal bones and muscles will be much more efficient and much faster. Instead, consider the creature feeding on small insects that land on it (perhaps attracted by it looking so water-like). Perhaps the creature walks into a relatively drier region, attracting more insects; after drying out over the course of the day, shrinking in size but having fed, it walks back to a water source and refreshes itself. This feeding mechanism would also go a long way toward explaining the evolutionary pressures that keep it looking water-like.
Finally, to explain the reason why it preserves the ability to become relatively liquid at will, rather than merely having an outer layer of jelly to attract insects, we can introduce some degree of predation. Since the way of walking is so slow, it relies on camouflage. The predators stamp their feet loudly to flush out prey, and they will see if something that otherwise looks water-like doesn't have ripples from the stamping. The best way to have ripples is to be a liquid. The creature will collapse into a puddle, and perhaps even push liquid water from inside to the surface to form an even better layer to show the ripples, re-absorbing it when the predator has moved on.
I think this could be possible if your creature was made up of multiple units capable of separating from a larger main body. For example, an individual ant behaves like a solid, but ants in a swarm behave like liquid crystals and flow around obstacles. If you were to apply this to an individual organism, the units would need to be a lot smaller in order to act like a liquid, and linked together as some sort of colony or hivemind. The closest known organism to what you're describing would probably be a slime mold, where cells can contract and expand through chemical reactions.
Nothing we know about biology supports a creature like the one you've described, so we're either talking about completely alien science and technology (and we all know the trope about "sufficiently advanced technology") or magic.
So, if we're assuming magic:
A bunch of small creatures, amoeba- or paramecium-sized, which (to hunt, perhaps, or to migrate when their water environment dries up) are capable of clumping together into a "creature" which walks over the land and dissolves into a cloud again when once again in water.
The evolutionary path to this creature is... unlikely, since a great many of the creatures would die in "land mode", but alien life can be "cool but implausible" if you wave your hands hard enough. (Energy and metabolism is also a problem.)
As an added bonus, it would presumably be somewhat difficult to kill, since you'd have to destroy most of its mass to kill the "land version" of the creature.
I don't think that a naturally occurring organism could transform itself from solid to liquid and back--unless you count animals that can survive being frozen solid and then thawed, which involves no water mimicry.
However, it might be possible to construct a robot that can impersonate a liquid.
Tiny robots, the size of grains of cornstarch, could move suspended within some medium such as water. If the flock of tiny robots could coordinate its own movements, what you would have is the equivalent of an animate blob that could flow like a liquid but sit like a solid--sort of like Ooblek. https://www.scientificamerican.com/article/oobleck-bring-science-home/
I'm not sure that water is the ideal medium, or what properties the liquid medium would have to have to construct the best robot of this type.