Looks like a regular ol bristle worm to me as well. Many people don't mind them because they are scavengers and eat all the garbage from your aquarium, but they DO have bristles that sting fish and corals. Some bristle worms are even venomous and cause irritation. Not something I would want in my tank and they can be hard to remove. I'd leave him out. Don't want them reproducing in there and over running the place.
Most likely there is already more. It is one thing if you find it on a piece of incoming rock/coral but if you found it in an established rock/tank there is probably more.