Bristle worm good. Just don't touch. The crab hiding could be a hermit without a shell. Leave a few shells scattered around him and see if he reacts. If it has'nt moved as i would suspect from your pics, it could just be an empty exo -skeletonBristle worm! Is that good or bad ? Shouldnt a crab have a shell ? Its just hiding in a crevice in the rock.