Diagnosis
That's the waveguide cover, a mica plate. It's not a stain you scrub off—it's often baked-on food splatter or, if it's dark brown/black, arcing damage from metal or foil.
How to Fix It
First, see if it's just grime. Put a bowl of water with lemon juice or vinegar inside, run it for 5 minutes to steam everything, then let it cool. The cover is held by one or two screws. Carefully remove them (the microwave must be cold and unplugged). You can gently wash the mica plate in warm, soapy water. Let it dry COMPLETELY before reinstalling.
If That Didn't Work
If the plate has burn marks, pitting, or holes, it's damaged and must be replaced. A damaged waveguide cover allows moisture and food into the magnetron area, which can cause arcing and destroy the magnetron. Replacement covers are inexpensive and specific to the model.
Heads Up
Never operate the microwave without this cover installed and completely dry. Also, never use abrasive cleaners or scrubbers on it, as scratches can lead to arcing points.