Take a blowtorch to the rock
(Actually I'm not really kidding - if you do it though be careful, you don't want the rock shattering on you.)
Do you have any dwarf/pygmy angels (Centropyge) ? They sometimes will pick at it.
The problem with just scraping them off is that they burrow into the rock so you only give them a haircut.
You can sometimes smother them with epoxy (you can always remove the epoxy later) but this looks like a pretty large colony.
I'm thinking blowtorch or letting the rock dry out and reseeding it in a curing tub are probably the better silver bullets.
Good luck..