Tiny tentacles protruding from a hole in the live rock, long thin legs peeking out from under your aquascape? Probably a micro brittle starfish, friendly serpent or brittle starfish. These creatures love leftovers and detritus.
These easy critters will thrive in a well fed tank. If running a Low Nutrient System, then offer these thin legs a tiny meaty morsel once a week. Have a fish that likes to eat stars? Add this critter to your refugium.