Are you a painting contractor?
I am licensed as a general contractor, but we do a lot of painting, both interior and exterior. We do not keep any inventory so when doing jobs, we buy what is needed and rarely store any leftover paint. I have stored some paint before in my basement, but have never had it go moldy, not the least bit, and of course have never seen anything like this, nor have I ever heard of any mold like this occurring. I have even stored paint for a year or two, or even three, and have never come across a mold problem before, which is why I am asking. I am not one to throw things away, and hate to throw away about $100 of paint. I guess it is the frugal workingman in me. LOL
My basement does not go anywhere near freezing as it is always the warmest part of the house in the winter, probably because that is where the gas furnace that heats the house is located. I am well aware that freezing temperatures can ruin paint.
The mold is white in color and is about a half inch thick layer on the entire top of the paint. There are also big round white mold spores growing on the inside of the lid of the bucket.