Salts and minerals leaching out of concrete basements or walls form a white, powdery crystals called efflorescence. Sometimes it may be difficult to visually distinguish between efflorescence and mold. However, since efflorescence is caused by water problem there are chances of mold growth too.
The easiest way to distinguish mold from efflorescence is to put some of the stuff into a drop of water. If it dissolves, it is not mold. Also, since efflorescence is crystallized salts, it breaks into a very fine powder if squeezed between fingers.