Are you using a municipal water system or well water? I would say it is lime deposits if using the first. Our water company adds a ton of lime to the water and it leaves a white powder when it evaporates. If you boil it you end up with white powder on the sides of the pot. IT is harmless, other than keeping the PH up. You can try filtering the water with some type of filter you put on the water faucet. It helps.