See How to clean Mac products for information about cleaning your Mac, display, keyboard, mouse, and more:
To clean the display on your iMac, first shut down your iMac and unplug the power cord along with other connected devices. Next, dampen the cloth that came with your iMac—or another clean, soft, lint-free cloth—with water only, and then wipe the screen. Do not spray liquid directly on the screen.
Don’t use window cleaners, household cleaners, aerosol sprays, solvents, alcohol, ammonia, or abrasives to clean the display.
Warning: Don’t clean the screen with a cleaner containing alcohol or acetone. Use a cleaner intended for use with a screen or display. Never spray cleaner directly on the screen. It may drip inside the display and cause damage.