This is an interesting invert...it is usually visible (not hiding) in my 29 gallon biocube. I tends to live under and on the sides of the live rock. I've noticed on a couple occasions it will grab a bristleworm and rip it in half. I'm not sure it eats them as I have seen parts of bristleworms floating in my tank. It almost appears to use the bristleworm as a weapon or self defense. When I took this picture, it stuck them out at me with a threatening gesture.
So far it has left everything else alone in the tank. I do think other small inverts could be at risk if they are too slow to get away though.
In nature, some species of squat lobsters live on feather stars and apparently steal their food. I'm not sure if this species is one of those or not.