This mainly occurs with the use of hard water (highly mineralized). Such water is inherently more alkaline and has a higher pH. Alkaline water has less potential to keep dissolved minerals in solution compared to more acidic water. Over time, the minerals start to precipitate and, depending on their composition, they will either sink to the bottom or float to the top. This process is called flocculation. It is harmless and is not a sign of a malfunction in your system. The white residues are minerals that were already in your water and are now simply visible, whereas they were previously invisible due to their suspension in ionic form.