i'm not sure if this would work but there's a thread on RC that pertains to getting rid of bryopsis with elevated Mg levels. for some reason it all just dies off within a very short period of time, might just work for GHA and give you a leg up. i haven't read the thread for a few months so i'm not 100% sure what new but it might be worth taking a look, it is a very long thread.
http://reefcentral.com/forums/showth...light=bryopsis
good luck and hope this might help...