DIAGNOSIS The white deposit is mineral scale from hard water. It is calcium carbonate and other minerals that the filter does not remove, accumulating on the filter surface as water evaporates.
FIX Step 1: Remove the filters. Step 2: Scrub the exterior gently with a Scotch-Brite pad or similar non-metallic scrubbing pad under running water. Step 3: For heavier buildup, soak filters in a 50/50 mixture of water and white vinegar for 15-20 minutes, then scrub. Step 4: Rinse thoroughly.
IF THAT DIDN'T WORK If scale is also forming inside the upper or lower chamber, clean the chambers with the same vinegar solution and a soft cloth.
HEADS UP Do not use metal scouring pads or abrasive cleaners. They will damage the filter's outer shell. This scaling does not mean the filter is spent.