Actually different kinds of mold can be different colors, that green stuff that grows on old bread, that ain’t algae, it’s mold. Bud rot mold is usually grey, but it can have hints of green in it too. The mold that grows on soil will most often be gray, but it can green or even white. The mold in his bag was probably from the soil mix being too wet/moist and the environment where it was stored a bit on the warm side. The mold spores could’a come from anywhere, even from the nursery the mix came from, or it could have already been residing in the closet maybe because of any of a number of reasons. I remember growing up in a home that was “swamp” cooled or cooled with a evaporation cooler. The humidity levels would get so high during our summer hot rainy season that the cooling affect would kinda stop working and leather goods stored in the closets, like belts, boots, leather luggage, etc. and such would often develop green mold growing right on the leather.