Diagnosis
The screen has a matte anti-glare coating that can be damaged by harsh chemicals or excessive pressure. Using the wrong cleaner will create permanent smears or strip the coating.
How to Fix It
Power off the monitor. Use a dry, soft microfiber cloth designed for lenses or screens. Gently wipe the surface in a circular motion. For stubborn smudges, slightly dampen a portion of the microfiber cloth with distilled water. Wring it out completely. It must be only barely damp. Wipe the screen, then immediately dry it with a separate dry portion of the cloth.
If That Didn't Work
For adhesive residue or oil, use a cleaner specifically formulated for coated LCD/LED screens. Spray the cleaner onto the microfiber cloth, never directly onto the screen. A 70% isopropyl alcohol solution can be used sparingly, but only if applied to the cloth first. Test on a small corner first.
Heads Up
Never use window cleaner, ammonia, acetone, or ethyl alcohol. Never use paper towels, tissues, or rough cloths. They will scratch the coating. Apply minimal pressure. The goal is to lift debris, not grind it into the surface. Clean only when the display is cool to the touch.