OK, so now that we have established what I already thought......
How does one "get" mold? I need to tell him something he can learn from.... The only thing I can think of is that he has the thing parked in a closet in the (slightly damp) basement, which may well have a lot of these critters flying around in it.....but how they got in the bucket I don't know, unless he opened it and they jumped in..... will have to ask.
(Never had this problem myself, yet....)