White spot on neon tetra

I had this neon tetra for while now and I noticed a white spot on him. Is it columnaris or NTD? What treatment should I do? Should I quarentine?

the white spot is on the top near the fin.

I got some other neon tetras recently and maybe they transferred something to this one?

Unless my eyes deceive me, I can't see the spot...?

If the fish has had the white mark for a few months and it hasn't changed, then it's a parasitic worm that got lost in the fish and now lives in the muscle on the back. The worm will live here until it decides to come out and might pop out through the side of the fish, or it might stay put indefinitely. You can try deworming them with Flubendazole but even if the worm dies, it will remain there.

If the mark gets bigger over a few months and the muscle tissue turns a milky white colour, then it's a microsporidian infection and can be treated with salt.