That's not a metal plate. it's heat resistant mica composite. The microwaves come into the oven through the hole behind it. You can take it out scrub it off, put it back in and be good to to go. Be gentle with the stuff. It'll crack if you scrub too hard. From the soot marks on the metal above the piece, you must have had a big blob of grease explode onto the shield, where it burned merrily. Clean the soot off too. Pull the piece up gently to remove, and push it down under the clips to put it back.