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Could be just a nip injury, with a little fungus. If so, just keep the water clean and it should heal on its own. Research Columnaris treatments if the white patch starts to spread. Good luck.
Look into lymphocystis...
It is viral and no treatment is available [like a cold for us]
It may just go away on its own..Possibly.
I would not sweat it unless you see some other symptom..