Anything change in lighting or flow? Are your params stable and what are they? Anything else looking bad in the tank?
Wow Matt.....in the second pic it loks like the top is browning.......and then it turns white........My guess would be lighting. How are the bulbs above it?
Has to be a disease man.....you can see it brown then turn white from your pictures.....i'd cut it and see if you can save it
Sorry Homie
FWIW I'd give it the smell test, if it doesn't pass, get the polution out of the tank. The necrotic tissue can pollute pretty quickly.