WHat you saw was probably a peanut worm, not a bristleworm. And bristleworms are good. THe only bad worm you really need to worry about [that you're likely to meet] is the eunicid worm, which has 4 tentacles around his mouth parts: if you spot one of those, get it out. Otherwise, value them. If you didn't have them you'd have to buy some, or risk a nitrate spike if you have something die in your tank. Bristleworms cannot harm corals or fish, physically cannot, give or take a faceful of harmless spines that fall off after a couple of days, if a fish tries to eat one: the only thing in danger is your fingers if you touch one.