Yeah, I don't know why Flatworm Exit didn't kill them.. But even a double dose didn't do it. Plenty of them died, but I saw plenty more 'weather the storm' so-to-speak and just glide across the glass like it was no big deal.
After 2 bottles of FWE I decided to go natural... The leopard wiped them out in a few weeks. YMMV, some wrasse will hunt them to extinction, some wouldn't bother. You know how that goes though.
As for the ghost worms themselves... At most they're unsightly, and make people nervous they'll get a pest from you. While technically, they ARE a bit of a pest... The don't do anything but exist and be annoying for existing in the first place. A lot of people don't realize they have them.