Nothing to worry about... it's just a big polyp. Corals aren't always "perfect"... not all their polyps are going to be exactly the same size. And occasionally they're going to have some that are a lot smaller or a lot bigger than the rest. They even get tumors and "cysts" and other such things. I don't think it's necessarily a new branch forming, but it could be.
I don't know if there's a good analogy with humans, but well, have you ever seen a person with like, one big fat, long hair coming out of his ear? Not a bunch of hair... just one that sticks out... or a big mole? It's kinda like that. It just happens... who knows why?