Interesting... although not clear, can see that something is affecting it.
I haven't encountered anything that willingly eats leather corals... most all fish and inverts won't even pick at them.
That said, what are your parameters?
Looks like part of it is just dying. Is there anything that could be stinging it? I don’t know of any fish or inverts that eat leathers. To me it looks like it’s just dying, they start to disintegrate when they die. What are your parameters and are they stable? What kind of lighting and flow? How old is the tank? Can you post a pic that is zoomed out or a FTS?