That looks like a virus infection I have seen before.
Some people call it pleco mouth warts. Some say it's related to lymphocystis.
But I think it is some variety of papilloma virus.
Afaik there are no cures for papilloma besides providing pristine conditions and letting the fish fight it on it's own. If it's papilloma there is at least a chance for recovery.
I would increase the water changes to at least 70% per week, add tannins to the water after each water change and feed only high quality foods.