Diagnosis
Polymerized food splatter. Standard wiping is ineffective. You need a steam-cleaning method to loosen the deposits without abrasive scrubbing.
How to Fix It
Place a microwave-safe bowl with 1 cup of water and 2 tablespoons of white vinegar inside. Run on High for 5 minutes. Let stand for 5 more minutes to allow steam to condense. The residue will wipe away easily with a soft cloth.
If That Didn't Work
For persistent grease, use a paste of baking soda and water. Apply to the stain, let sit for 15 minutes, then gently scrub with a non-abrasive sponge. Wipe clean.
Heads Up
Never use commercial oven cleaners, steel wool, or abrasive pads. They will permanently damage the interior coating. The door seals must also be wiped clean to maintain proper closure.