Ok, it's not a bristle worm. (That's what you asked us to do ).
After i first got my tank, I researched the worms and read a number of different articles, and they all said they were beneficial. The bad thing about them, is that if you touch them, their bristles stick in your skin, just like a cactus, and hurt a little. the only ones that are undersireable, mainly because they nip on corals, are Fireworms, and are a deep red, with much larger bristles. I have a ton of regular bristleworms, and they come out whenever I feed, almost immediately. The big ones, and some are at least six inches, will snag a shrimp pellet and drag it back into the rocks. I thinks it's neat to watch.:happyfish
