It looks like it could still possibly be lympho or maybe argulus (fish lice). The odd thing about it being argulus is that one, there's only one parasite and two, the salt didn't work. I'm leaning toward lymphocystis mainly because he's still acting normal. It's possible the spot doesn't look cauliflowery because it's quite small.
If you want to be on the extra safe side and try a medication, I'd suggest either General Cure, Hikari HealthAid PraziPro, or Seachem ParaGuard. Those are all effective for external parasites which, next to lympho, is the most likely cause of a growth/spot like that. But there's also the possibility it's a tumor too. I'd say if the spot doesn't change at all (either growing bigger or growing smaller) after a few months, it's a tumor.
I am pretty sure it's not an ulcer or sore associated with columnaris or fish TB, though. If it was, his behavior would be different and the spots would be growing and spreading.
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