There are different kinds of bristle worms so it is hard to tell you for sure if it is a good one or bad one. 90% of the bristle worms we find in our tanks are harmless though and good to have because they are detrivores
Here are a couple of links to help identify hitchhikers
http://www.xtalworld.com/Aquarium/hitchfaq.htm
http://www.melevsreef.com/id/