bristleworms

[Keen]

here is a comment i found on a site
Primarily it is the Pherecardia (Pacific), Hermodice (Caribbean), and other related species that one needs to be concerned about. These Fireworms are errant (roving) carnivores that can cause much damage in a reef tank. They bare toxic bristles on their bodies that can inflict a sting of a very painful degree, posses strong jaws for feeding, and can reproduce quite rapidly.

They are not selective about what they eat, but usually prey on all types of other motile (moving) and sessile (attached or stationary) invertebrates, such as corals, crustaceans, mollusks, sedentary type as well as other errant type bristleworms. Being particularly aggressive predators, they may even eat small fishes, if an opportunity presents itself.

[CMarty01]

I believe there are about thousands of types of bristleworms, so its really going to depend which ones you get. Some will destroy your reef and others will help clean up uneaten food. I'd say if you find corals or snails being eaten then set a trap and get rid of them, otherwise leave them be.