There are about a zillion species of 'bristle worms'. most are very beneficial, a few are questionable, a couple are probably bad, and a bunch are somewhere in between.
Mostly, though, it's quite unusual to find the 'bad' ones in an aquarium these days. They aren't impossible to catch, but it's pretty much impossible to eliminate the entire population (if you decide you want to).
Unless you see one actively eating a zoanthid or softy, I wouldn't worry about it. Even if you do, it's worth a second look to see if it's actually eating that or some crud around it's base.