Do you have an emerald crab? I never considered it, until I caught mine Green Clawed peeling the skin off my Duncan’s one night.
You may want to frag this above the tissue loss line to try and save it if you can find the culprit. My crab went to the Sump and the Duncan recovered.
Unfortunately not a good update. I moved him down to the sand bed and tried a couple different spots with shelter but it just seems to keep spreading. Everything else in the tank is doing well, and numbers are coming back the same as always. My temp is a little high but other than that all is the same. Not sure it's going to pull through.