some bristle worms are beneficial but MOST ARE NOT, if you have any coral or other inverts in your tank it is highly advisable to remove them ASAP as they multiply rapidly and in large numbers can and will kill corals and other inverts, your local pet store should sell a bristle worm trap about $10 or you can make your own, or there are several fish that eat these pests, Strawberry dottybacks are great for this but if the worm is over 2 inches a trap is the only way to go, I highly recommend getting rid of them,