screen water damaged on macbook air

I haven't tried the following on coffee but may be worth a try. See the following from How to clean your Apple products - Apple Support

When cleaning the outside of your MacBook, MacBook Pro, or MacBook Air, first shut down your computer and unplug the power adapter. Then use a damp, soft, lint-free cloth to clean the computer's exterior. Avoid getting moisture in any openings. Don't spray liquid directly on the computer. Don't use aerosol sprays, solvents, abrasives, or cleaners containing hydrogen peroxide that might damage the finish.

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.

To clean hard-to-remove smudges or fingerprints on the display or exterior of your Mac, you can use a cloth moistened with a 70-percent isopropyl alcohol (IPA) solution to gently wipe the display or enclosure of your Mac laptop.

MacBook Pro models from 2016 and later with Touch Bar and Touch ID: Clean the Touch Bar and Touch ID (power button) on these computers the same way you would clean the display. Shut down your Mac and unplug the power adapter. Dampen a soft, lint-free cloth with water only, then use it to clean the Touch Bar and Touch ID.

MacBook (13-inch, Late 2009) and MacBook (13-inch, Mid 2010): The bottom case of these computers uses a soft nonslip material. Use a 3M Gray Microfiber or soft dye-free, lint-free cloth to clean the bottom case.

Is the stain on the screen the only damage your MacBook Air sustained? If so, you're lucky!

I've used a product called Whoosh to clean my MacBook Air's screen. Its manufacturer claims it's used in Apple Stores (though I've no way of confirming this). Although I only rarely have to use it (I normally just use a microfiber towel dampened with water, followed by buffing the screen with a dry microfiber towel) it does seem to work better on stubborn stuff.