Looks like a bacterial infection, could have come with it if its new and stress of acculamation let it take hold, the clown fish could have irritated enough to cause it, though that is ususally not the case. Euphylla: elegance corals, frog spawn and torch corals are more likely to develop bacterial infections, I do not know the reason why, maybe some one else can provide that answer. I would remove it from the tank. That just my opinion