This is calcium carbonate, which forms from the minerals in the water. It is normal that the minerals can precipitate out of the water. 100% of your water content is flowing into the unit, but the minerals are concentrated into the Alkaline water, which is only 2/3 of the water coming out. This higher concentration has increased the chance for them to precipitate out naturally and they will bond to the glass. The calcium is not harmful to you and can be easily cleaned from glass using vinegar.