DIAGNOSIS You're right to be cautious. The 27UK850-W has a semi-matte anti-glare coating that's more delicate than a standard glossy screen. Harsh chemicals or pressure can smear or damage it.
FIX Use only a dry, soft microfiber cloth (like the kind for glasses) first. Gently wipe in one direction. For smudges, lightly dampen a part of the cloth with distilled water—just water, no additives. Wring it out completely so it's barely damp, wipe the screen gently, then immediately dry with another dry part of the microfiber cloth.
IF THAT DIDN'T WORK For stubborn marks, a tiny drop of isopropyl alcohol (70% or higher) on the cloth, not the screen, can work. But this is a last resort, as repeated use can potentially degrade the coating over many years.
HEADS UP Never use window cleaners, Windex, anything with ammonia or acetone. Never spray liquid directly onto the screen—it can seep into the edges and damage the electronics. The coating will show more 'sparkle' or grain under direct light when clean; that's normal for this type of panel.