Light grey shade at the top left corner of my iPhone 6 screen

I just noticed a light "blemish" on the top left corner of my iPhone 6. I'm not sure if it's a dead or stuck pixel. But I wonder what caused it. I'm not even sure if it was there since the day it was given to me. I got my phone at January 2, 2016.

The light gray thing is not really visible in white background, but with colored background it's visible. :/

Imagine all the 4 corners of the iPhone intact with color except the one at the top left which has a small light-ish gray shade that is visible in a colored background.

Space Gray iPhone 6, 128 GB, iOS 9.2.1