Fungus on the body appears as cotton wool-like growths, it is hard to tell if a Koi has it in the gills, but if it hangs at the surface gulping for air it is likely.
Carp pox. A virus that produces solid waxy lumps on Koi. It will not kill Koi and is generally harmless, but can look unsightly. It is most often present in small Koi and in cold weather, clearing up and disappearing when Koi grow and in the spring when water temperatures rise.