Have you tried investigating it???? touch it, if its limp and wavy its probably some kind of macro algae as suggested before, if its soft and somewhat limp it maybe some kind of wierd sponge, If it's hard it may be some type of SPS that had be laying dormant on the rock and finally found the proper conditions to start growing faster. Don't quote me on that last one, I don't know much about SPS so I don't know if thats even possible, but its an idea.