Bristle worms are very Beneficial , they keep your tank clean from detritus also there one of the fist lines of defense if a fish dies & you don't notice theses little guys will be cleaning it up quit quickly depending on the number of them
they are good as long as their numbers are low, when they over populate they will steal food from your coral's mouth. Whenever I see one I use my tweezers to take them out of the tank and flush them. This is the only population control I have on them and they seem to be growing in numbers...