Help! Weird white thing on Neon Tetras mouth! i thought it would go away, it has had it for a few weeks now.

it has not infected any other fish

Looks like the virus? that affects neon tetras. We think it's a virus and it's been appearing on neons for the last couple of years. It causes a white lump to grow and eventually the fish can't eat properly. The fish end up being euthanised.

There is no cure.

It unfortunately does look like what Colin T said to me too. It almost looks like the jaw is inside out when they develop it, though I can't see in the photo if that's the case here.
Sorry to be a co-bearer of bad news, but it is bad news for him.