This started showing up on one of my Peacock Bass. Appears to be on end of the tongue and/or the tongue is curled up. He's about 7 inches and had him since he was very small. No changes to the tank in a long time with weekly water changes.
He is eating, diet is frozen raw shrimp and pellets.
I've seen something similar on my pike's lip area at one point. It disappeared after a couple of weeks, so I surmised it might have been a wound cause by lip locking or biting on something hard like a pellet.
You could take the fish out, and gently rub the area to see if it's just "skin".