Have there been recent changes you can see in your journal? As I said, the phosphates are high, but I wouldn't see that being the issue like that. Most of the tissue is dead and gone, but the rim is still there.
Just don't take it out. Do water changes to see if you can dilute whatever the issue is in the mean time. I had an orange monti 'die' when my AC
broke and it went over 100 in my living room many years ago. Nuked most of the coral in the tank, but once things were back under control, the monti's eyes showed their color and the tissue regrew.