I had a ton of these worms in my tank, Even one that was over 12"s long and THICK which scared the hell out of me.
they left everythign alone till I put in plate type corals, and at night they went to town, killed one, and maimed another. So I went bristle worm hunting and got an arrow crab to help keep them down. I still have a few, but My plate no longer gets attacked by dozens of them.