It looks pretty pink, so I doubt it's fungus. But if it is an injury then there is a chance of secondary infection. If your water stats are good and have been for the months running up to this, then I would continue the good water quality and watch to see if feeding habits change. If you have a mature isolaton tank, that's handy. But if not then dont rush to seperate just yet. Lymphocystis can't be treated but can spread to other fish, however, this could really be anything, and it's suddenness and colour suggests injury to me. Inspect your fish for signs elsewhere on its body that could give us more of a clue if its bacterial/fungal.
Ive personally had more success not seperating a fish and using stress-reducing water treatments and salts then I have using medication in an isolation tank.
