It is probably a bacterial infection, either Pseudomonas or Aeromonas. You can try JBL Furanol or Waterlife Myxazin. The "yellow powder" thing sold in small packets with Japanese text on them works too, but do not overdose. Probably better to use something with English text so you know exactly how much to dose.
This seems to be confined to just the particular spot on the cheek. I'd suggest you recheck your other fish in the tank. Likely to have been introduced by one of your newer fish, or any feeders you might have used.
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