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I am assuming you are talking about bristleworms.  Bristleworms are detritivores and will eat any food that your fish miss.  They can be beneficial to the balance of a coral reef tank.  Most species of bristleworms we find in our reef aquariums are completely harmless unless you touch them.  Their furry little spikes can get embedded in your skin and cause an annoying irritation.  If you have a population explosion and you would like to keep in under control, try Coral Banded Shrimp (Stenopus hispidus), or Arrow Crab (Stenorhynchus seticornis).

Good Luck!