Diagnosis
VA panel backlight uniformity issue, often called "clouding" or "dirty screen effect." This is a physical trait of the panel and backlight assembly.
How to Fix It
Adjust the local dimming setting. In the OSD, go to Picture > Local Dimming. Set it to Auto or High. This dynamically controls the LED zones to improve black uniformity. Also, reduce the Brightness setting. High brightness exacerbates clouding.
If That Didn't Work
Gently massage the affected area of the screen with a microfiber cloth. Use very light, circular pressure. Sometimes the panel layers can have minor separation causing shadows. Do not press hard.
Heads Up
All curved VA panels have some degree of this. It's most visible in a dark room on a full-screen black image. If it's severe with local dimming on, it may be a defective panel.