If it is white, it could possibly be a bristleworm that are detrivores and eat all of the waste and food that drops. They come out at night, but can be enticed with any food that has garlic in it. Don't touch the bristles if you take them out. They will swell up your fingers so that you can't move them. I have loads of bristleworms in the tank of several different sizes. They live in the rock and don't bother much.
The fireworms tend to be reddish on the bristles and are not good. Google images either bristleworm or fireworm and see the pictures