Try iodine solution (the one for human wounds) applied with a Q-tip.
And if you have the guts for it, try to trim it back as much as possible.
From all I've seen so far, if it gets this big, it's pretty much a losing battle.
I've had fish that had the "goat-beard" like looking version at the bottom of their lower jaw and that one, once stopped in growth, seemed to be benign and the fish lived with it for years.
But the infections that hit primarily soft tissue seem to be life-threatening and difficult to get under control.
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