Facts about Bristleworms per Aquarium Corals by Eric Borneman
Bristleworms are primarily beneficial detritus eaters, but larger specimens of a few genera may be carnivorous (and would need to be removed). Bristleworms are also highly reproductive, are regenerative and may reach high densities in the aquarium. This is not necessarily problematic because their larvae provide an important food source for corals and other filter-feeding invertebrates.
Smaller bristleworm specimens can be controlled biologically. The Red Sea dottyback fishes seem fond of the smaller worms [see the author's book on page 396 for a complete list of other fish that feed on bristleworms as well].
No reef is ever likely to be completely free of bristleworms, nor are they usually harmful to corals.