looks to me like a bristle worm- they're fine - eats detritus and such, be careful handling them- those "hairs" can be itchy if you get stuck by them- if you do, eith lay duct tape or similar accross area making sure its stuck on well and peel off- the tape should pull the bristles out of your skin- if not, soak affected area in white vinegar to disolve them.
Keep it, great detrivores, and unlike some propose by their hasty responses here, they are not going to sting you every time you stick your hand in your tank. If you grab them, they do have bristles on their bodies, but in my 4 years of reefkeeping, I haven't ever been stung bad enough (by a worm anyway) to warrant a negative response.
