Efflorescence is a white powder that results when mineral salts in mortar are dissolved by water. These deposits usually disappear after a few years. If you want to get rid of them now or if you have other stains or problems, try the techniques listed in the chart below. As a general rule, try common household cleaners first; use acid only if necessary. Do not use acid on limestone or marble. Caution: Always pour acid slowly into water; never poor water into acid.