When the minerals in concrete, mortar or soil dissolve in water, they can end up as deposits on the surface of a wall or floor. When the water evaporates, it leaves the minerals behind. This mineral residue manifests as white powder or crystals know as efflorescence. It is sometimes confused with mold, but it is not alive. Efflorescence is not harmful unless you eat it, and it can easily be swept or brushed way. It is however a sign that water is slowly coming through the concrete and a good reason to call a professional to inspect the foundation.