From what I have read it is usually a coin flip. I asked the LFS
about a Flame Angel and they said that 80% won't bother coral if they are well fed. I asked another LFS
and they said its 50%-50%.
One helpful employee at a LFS
suggested purchasing a specific kind of inexpensive coral (she stated the kind but I don't remember) and placing it in the tank. She said dwarf Angels are drawn to it and will usually only eat that specific coral and leave the others alone.
I don't know if this will actually work or not.