Hi - I had the same problem. It's mouth fungus and it's awful. I got one comment when I posted about it several years ago. Here's the comment:

"The first thing you should always do when any of your fish gets sick
is make sure all your water conditions are in excellent shape. Do you
have a liquid test kit? If so do an ammonia, nitrite, and nitrate test.
Ammonia should always be at 0, nitrite 0, as well, and your nitrate
below 20, but preferably below 10. If you do not have a water testing
kit, you really need to buy one, and the liquid work best, the strips
are inaccurate. If you don't have one though, begin by doing a 50% water
change, to ensure all medicine is out of the tank, and the water
conditions are good.

"For further treatment you should get the medicines primafix and
melafix. Treat until you see an improvement (this generally takes about a
week). Do a 50% water change once a week until the fungus is gone, and
be sure to invest in a liquid test kit. API master test kit is one of
the most affordable and best working."

I followed instructions but my poor guy passed away. I hope you have better luck!

Primafix and melafix gunk up the water and cause stress to the fish. Don't use them.

I miss thegab (goldfish and aquarium board)

Edit: didn't realize this was such an old post, my bad. Is there an update on this sick fish?