DIAGNOSIS That's called backlight bleed or VA panel glow, and it's a physical characteristic of the panel type, not a defect you can fix. The CQ27G2 uses a VA panel, which typically has less bleed than IPS but can have some clouding, especially in the corners due to how the panel is seated in the frame.
FIX There's no software fix. First, assess it in a dark room with a full-screen black image (search 'black screen test' on YouTube). If it's minor, try gently massaging the affected corner of the bezel with your thumb. Sometimes pressure from the frame can cause it, and a slight flex can redistribute it. Don't press on the screen itself, just the plastic bezel edge.
IF THAT DIDN'T WORK Adjust your viewing angle and brightness. VA glow is angle-dependent. Make sure you're looking at the monitor straight on. Also, running the monitor at very high brightness (like the default 80-100) makes this much more noticeable. Lower your brightness to a comfortable level, usually between 30-60 for indoor use.
HEADS UP This is generally not covered under warranty unless it's extreme (like bright white spots or large patches). Taking a photo of it is tricky because cameras often exaggerate the effect. If it truly bothers you during normal use (not just on solid color test screens), your only real option is to exchange it early on, knowing the next panel might have it in different spots.