Is this a mold?
My apologies, but I cannot positively identify this. It doesn't appear to be mold. Another option could be efflorescence, which is often confused with mold. Efflorescence is an odd phenomenon I don't know much about, but the image matches with other examples of efflorescence. Efflorescence is essentially salt deposits that are whitish in appearance, and occur on masonry, stucco, brick and concrete.