Efflorescence is a white powdery substance on the surface of concrete or brick. Efflorescence is not mold. Efflorescence is evaporated salt caused by mortar and/or concrete or brick itself. The white powdery residue can be removed with a stiff brush (or pad) and a mild acid solution. There is no guarantee that this will not reappear or recur as the salts within the substrate itself try to work through to the surface again.