In addition to feeding on left over food small crustaceans, and detritus from which they extract uneaten parts, bristle worms attack corals and sometimes other animals too (anemones are a good example of the latter). Because of the damage they do, they absolutely need to be physically removed from the aquarium.
Are bristle worms bad?
Bristleworms may look ugly and a little creepy, but most are actually good for your tank—if they are not the poisonous type. … Bristleworms primarily are scavengers and consume uneaten food, detritus, and carrion in a saltwater aquarium.