Looks like a bristle worm to me as well. They can be good or they can be aggressive and not good at all. From what I remember reading, smaller ones can be OK and get more aggressive as they get larger, looking for more food sources. It also depends on the type of bristle worm. I have heard there are ways to trap them, do a Google search for it. Personally, I would leave it unless I were to see issues with my fish and corals.