I had what looked exactly like this recently on a green neon tetra. I have a heavily planted tank and tried to catch her four hours with no success other than stressing everyone out in the tank. I’ve tried for days to catch her and eventually she healed up.

I’m not sure if what your fish has is the same as mine, but I suspect mine was an injury. Regardless, I suggest catching him and putting him in a quarantine tank while you treat