White mold on concrete block

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I live in a slab house with one round of concrete block showing on the outside of the house just below the vinyl siding. The bedroom is the only room of the house that does not cool, summer or winter, and the floor is cold. There is a white powdery "mold" on the concrete block at the bedroom ONLY. What is it? My son-in-law says it is calcium leaching through, but why just the bedroom? What is it?