PewPewPew & Eevee,
"The sodium chloride is aquarium salt"
The good news is that it doesn't seem to be an bacterial infection, as there is no reddness or oozing around... whatever 'that' is.
Have you tried brushing the area gently
with a q-tip, to see if whatever it is attached to him, part of him or just something stuck to him? It almost look like a piece of sticker stuck to him... which makes no sense, but weirder things have happened...
My other though was an external parasite, but again, I don't see any swelling, redness or other sign of bodily penetration.
PM OFL is a good idea, for sure...
Good luck! I hope it's something minor. :)
He's a lovely betta boy.