It's a chemical reaction...moisture is actually pulled from the air and water droplets begin to form. When it gets humid in the summer I've even seen the salt creep get "wet" from the moisture. Wherever you spill salt mix, you will see condensation if there is enough humidity. If I remember from chemistry, it is the calcium carbonate that does it (I see moisture all the time in the measuring spoon for my turbo calcium) You should still be able to use your salt providing it's not a solid block, so I wouldn't throw it out. But I do agree with testing it. If the calcium levels are a little low, just add some supplemental to the tank.