Once a nipper always a nipper, there is no way of stopping them once they start nipping or get a taste for coral, thats it! If you don't care about the corals getting nip, keep the Angel. But if you like your corals more than the Angel, catch it and trade it or give back to your Lfs for another fish.
It was just a frag that broke off another coral (because it was getting picked at). What ever it is it must be tasty. The angel never nipped at anything else I have. I pretty much only have mushrooms though, mabye he doesnt have a taste for them,yet.