Can anyone id this worm? it's about 1/8" thick, white on the bottom(?) dark on top. The mouth completely retracts into the "neck" (excuse my french, but it's as if the head has a foreskin). The mouth has many white tenticles/teeth(?). The worm stretches out exposes it's mouth, touches the rock, and retracts it's head. It doesn't seem to be harming anything, just courious. In the picture you can see the head extended on the star polyps with the main body back behind the rock on the left. (sorry for the bad focus, it's dark in the tank :) ).