Diagnosis
That's biofilm or mineral scale buildup on the inside of the clear housing. It's common, especially if you have hard water. It doesn't affect filtration but looks unpleasant.
How to Fix It
During your next filter change, clean the empty housing with a mild solution of 1 part white vinegar to 3 parts warm water. Swish it around, scrub gently with a bottle brush if needed, rinse thoroughly. Never use harsh chemicals, bleach, or dish soap, as residues can damage the new filter media.
If That Didn't Work
For stubborn scale, let the vinegar solution sit in the housing for 15-30 minutes before scrubbing and rinsing.
Heads Up
Cloudiness can also be microscopic surface scratches from cleaning with abrasive pads. Use a soft cloth or brush. The housing is food-grade plastic, but it will scratch over time.