DIAGNOSIS That's not a stain, it's the fluorocarbon coating on the interior walls starting to break down from heat and splatter. Once it gets rough and discolored, it traps grease and is nearly impossible to restore to white.
FIX For general cleaning, use a soft cloth with warm water and mild detergent. For stuck-on grime on the coating, a paste of baking soda and water can help. Gently rub, then wipe. Do NOT use abrasive scrubbers, steel wool, or harsh oven cleaners, as they will permanently damage this coating.
IF THAT DIDN'T WORK If the coating is already peeling or severely pitted, cleaning won't fix the appearance. The coating is a non-stick layer over the metal cavity; its degradation is cosmetic and doesn't affect safety or function unless it's flaking off into food.
HEADS UP Yeah, this one's common on Panasonic models with this interior. To slow it down, always cover your food. The steam and splatter from uncovered items accelerates the breakdown of that coating.