When a salt loses mass (water of crystallization) when exposed to air, it is said to be efflorescent. 

Efflorescence refers to the spontaneous loss of water of crystallization by a hydrated salt when exposed to air. This occurs when the vapor pressure of the water within the salt's crystals is higher than the partial pressure of water vapor in the surrounding air. Consequently, the water molecules leave the salt crystals, causing the salt to lose mass and potentially become powdery.