Seashells,
You may want to try the cover up route with epoxy first, however if there are centralized to one or two rocks in the tank for what it's worth-
pull the rock! Mine were spread in little patches throughout my entire reef so I had to rip down the tank. These things can take over large colonies of coral with powerful little stings. Oh man your giving me flashbacks
The crappy thing about these worms is they can spread really fast in ideal conditions, so don't waste too much time on the matter.
Goodluck!