Eye Fungus on Koi

Hello,
My koi that I have had for 13 years has some white fuzzy stuff growing
from the folds around the edge of one eye socket. I have read about
Formalin and Malachite Green and treated the entire pond every other
day for 10 days. The eye appears to be the same. All other koi and
goldfish in the pond seem fine. The affected koi has become more
"shy", not wanted to eat food unless it floats away into a more
private place. I think he feels fulnerable to the other fish, and/or
cannot see very well. Any suggestions on what to do please? Thank
you Very Much in advance.