Wish i could send you a little of my DC to try but mail restrictions on liquid.
I think that might be stained in, and underneath it? Maybe mild abrasive like toothpaste on a felt drill mop? Or very fine scotchbrite pad or magic eraser might help if you are open to using those and clean well after.
Or
Prevention is better than cure, sell this Mighty and buy another, or a Mighy+ or Crafty+! And use it with capsules only, and/or dry mop with cotton after each use.
And make sure your CU is never dripping with brown oils! I don't think the large liquid pads help either.
Just some ideas for you.