Looks and seems to moving at the speed of columnaris?
I remove and euth. the fish.
Regardless of accurate diagnosis If it dies in the tank the you have promoted the spread of it to other fish. Even if another fish picks on it now the issue may be spread. Fish always find the weak and pick on them....
It could be from a infectious issue and other fish will show with it eventually or possibly just old age and only effecting/infecting only one .