Diagnosis
The matte anti-glare coating is delicate. Using standard glass cleaner or a rough cloth will create permanent micro-scratches and smears.
How to Fix It
Power off the monitor. Use a dry, soft microfiber cloth designed for eyeglasses or screens. Gently wipe in one direction. For smudges, dampen a second microfiber cloth with distilled water only. Wring it out completely so it's barely damp. Wipe the screen, then immediately dry it with the first dry cloth.
If That Didn't Work
For stubborn spots, use a screen cleaning solution specifically labeled as safe for matte, anti-glare LCD monitors. Spray a tiny amount onto the dry cloth, never directly onto the screen.
Heads Up
Never use ammonia-based cleaners, window cleaner, vinegar, alcohol, or compressed air. These will strip the anti-glare coating. Do not apply pressure; let the cloth do the work.