I am sorry your little one didn't make it, and hope everyone else in your pond is well.
I had a shubunkin that developed white and green fuzz as a secondary infection on an injury. It is called saprolegnia (sp? pretty sure I butchered that) which is always in a pond, but normally doesn't hurt strong, healthy fish. Kind of like how staph bacteria are everywhere, but only cause people staph infections if they have an injury, etc. It will come on if the fish is weakened or injured. I had to treat it with malachite green to cure it. Worked great, and the fish recovered fully. Might be worth looking up and seeing if it sounds like what your guy had.
Jen