This happens mainly with the use of hard (highly mineralized) water. This water is basically more alkaline and has a higher pH. Alkaline water has less potential to hold dissolved minerals in solution than more acidic water. The minerals then begin to precipitate over time and, depending on the mineral composition, will either sink to the bottom or float to the top. This process is called flocculation. This is not dangerous and is in no way a symptom of a failure in your system. The white residue is minerals that were already in your water and are now simply visible when they were previously invisible due to their suspension in ionic form.