Regular bristle worm. Quite a few of them in my tank, they keep the rock absolutely pristine. They won't hurt or eat anything in your tank unless it's already dead, and they keep things clean. They're light-averse, so if your lights are on and bright you probably won't see them. They'll also eat Formula 1 food pellets, after I set up the tank and cycled for 3 weeks or so I tossed in a few pellets and they all poured out of the rocks for it, they'd been starving for a while. They came out in full daylight, so I knew they were hungry. If you see any more, don't flush them. Just don't touch them without gloves, the bristles will irritate the skin.