Hello Fish, your issue is a very easy one to solve. It would be even easier if you can share a detailed FTS and a macro pic of the areas in question. I agree with all of the suggestions thus far with the exception of killing the GSP.
I don't consider any palythoa to be a nuisance simply because it is doing what it is innately predepossed to do, grow and flourish/propagate. Fragging if just a few polyps is an option, or removing the entire rocks for sale or trade. There's another option providing you can provide a picture. Whatever you decide, please do not opt to kill perfectly healthy polyps.
I would also suggest this link below and why I feel spacing is so important and yet never discussed. I see tons of frag tanks loaded with tiny frags all placed with totally improper spacing. If and when those frags ever begin to grow as your polyps have, they will have issues they never dreamed of.
http://www.reefcentral.com/forums/showthread.php?t=1502607&highlight=spacing
Good luck my friend.
MUCHO REEF