What to do about keyboard protector grime left on MacBook Pro screen?
Cleaning is easy. Follow the steps Apple recommends. 


https://support.apple.com/en-us/HT204172


For glass, that means 


  To clean the screen on your MacBook, MacBook Pro, or MacBook Air, first shut down the computer and unplug the power adapter. Dampen a soft, lint-free cloth with water only, then use it to clean the computer's screen.


The 3M gray microfiber cloth is a great option if you don’t have lint free cloths around. 

Anything else risks damaging the surface coating, which is what you likely have. The physical rubbing first erodes the anti glare covering and then if it continues, etches and erodes the glass itself. 

No solvent or cloth will repair that damage, just mask it temporarily with some types of glass cleaners. 


https://www.3m.com/3M/en_US/company-us/all-3m-products/~/3M-Glass-Cleaner/


These can damage hardware so use with sense and caution. Depending on your damage, these may make the glass look worse.