some species of bristleworms are poinsonis and can be preditory. however, because there are so many, it is hard to get an exact ID. But there are so few that are ever seen in the aquarium trade and they are also very rare. The one you have there is most likely a beneficial scavenger. he eats any extra food around, just acts as part of yoru clean up crew. They will only be a problem if you are keeping something like a tridanca clam. They will burrow into it through the foot and eventualy kill the clam