Diagnosis
A sub-pixel is stuck in a permanent 'off' state. This is a physical manufacturing defect in the LCD panel.
How to Fix It
First, attempt a software un-sticking method. Run a dedicated pixel-exercising video or website that rapidly flashes full-screen red, green, blue, and white. Run this for at least one hour. Second, apply gentle pressure. With the monitor on and displaying a white background, use a rounded stylus capped with a soft cloth. Apply very light pressure directly on the stuck pixel for 5-10 seconds. Release. Repeat a few times.
If That Didn't Work
The pixel is permanently dead. Check Dell's official pixel policy for your region. For the P2722H, Dell's Premium Panel Guarantee typically requires more than 6 bright sub-pixels (e.g., 2 full pixels) for a warranty replacement. A single dark sub-pixel often does not qualify.
Heads Up
Do not use excessive pressure or sharp objects. You risk damaging surrounding pixels or the anti-glare coating, which would void the warranty. Document the pixel's location with a macro photo in case you need to pursue a warranty claim.