Why do my leather sandals get a white "bloom" on them?

I've only noticed this the past 3 or 4 years. I have 3 pair of leather sandals that develop a whitish, kind of greasy-feeling "bloom" if I don't wear them for a few days. Looks kind of like salt stains. It wipes right off, although when it's built up over the winter it takes some rubbing to get rid of. There's no odor to it. These are heavier, slide sandals with rubber or synthetic soles; one pair by Dr. Marten, one by Skechers, not sure about the other. They're my everyday shoes in warm weather. I live in Michigan and my house is not especially damp or warm; nor do I have any foot problems like excessive perspiration or athlete's foot. This doesn't happen to other leather footwear... just these sandals. Sure would like to know what this is!