Well I'll be. My grandmother used to put stained tableclothes out
in the snow to "bleach" them. They always came out perfectly white.
Was it "snow mold" that did the trick?
I do not think so. Snow mold is a fungi which exist in organic
compounds and gets triggered, when conditions are right. When
organics, like leaves are covered by snow, and when snow starts
melting, that where it usually happens.
What you describe is something else. There are a lot of
micro-organizms inhabit snow. What we call stains, is a food for
them. They simply eat the stain away.