Mildew happens when clothes are in damp, warm conditions--and seems more likely in darkness.  For example, clothes at the bottom of the laundry basket or clothes that are wet with perspiration can mildew quite easily if not washed promptly. Mildew shows up as a fuzzy growth on the surface and between fibers. It can grow in a fairly short time -- not minutes, but hours or days. Mildew on clothes is often black, but it can also be white, gray, green, or rusty looking.