Until you get a better look at it, guessing about identity is a challenge.
One thing strikes me though....I don't find that most bristle worms move very quickly. They kind of crawl along pretty slowly. Eunicid worms, however, retract back into their tunnels like lightning! So you might have one of those.
If it is, some would say that they are problematic. (Good luck catching it!) I have many of them in my small tank and haven't found that they do any damage so far. Some can achieve great size, but others never get very large.
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