Do you have another tank you can move it to? In every case I have seen involving a tongue coral, something has been irritating it, they are very sensitive, I have heard stories of hermits and shrimp hanging on them destroying them over time. I'm not sure how large yours is but I was able to use a plastic strawberry basket over mine weighted with a rock and it did recover. Depending on size you could use a pvc ring to protect it.
Looking at it, I would say with confidence that something is irritating it physically. I wouldn't frag it......yet.
I would do something and see if the area stabilizes. Personally, I would use iodine on it, can't hurt. If the area didn't stabilize, I would frag off the dead area, being generously away from the effected area.
Sorry, it's a nice piece.