It's a stuck pixel. If it's just one pixel, Apple almost certainly won't take it back.
There are a few programs out there that can be used in an attempt to unstick the pixel. A few people have had success with gently massaging the area where the stuck pixel is, but you'd have to be very careful not to damage the screen in doing so. Fortunately, I haven't the need to test any of these remedies, so I can't provide much more detail than that.