Secondary infection?

[Diana]

That white fuzz on dead things and organic material (dead fish, food) is a type of fungus that decomposes stuff. There are probably several species. Most will only attack live fish at a site of a wound, where there is dead flesh. It is almost never a primary infection. It grows white at first, then picks up debris and can look grey.

http://www.nationalfishpharm.com/fish_d ... rders.html

Here is a link to a site with lots of disease information. Maybe it could help?