Green Pixels appearing on iPhone 6S Screen, and spreading

I have been using my iPhone 6S for close to 3 year now without any issues, and its a great phone overall. Yesterday morning I saw a small Green Color pixel on the top of the phone and since then it has been spreading. Not sure what this is, I contacted Apple chat support and shared the images of the screen but even they don't any clue about this issue.

I haven't dropped the phone, or caused any sort of physical/water damage.

Posted and image I took from another phone and additional the images of the increasing issue with the Screenshots on the imgur link https://imgur.com/a/vFxg6k8

I'm planning to go to the apple service center over the weekend.

Does anyone else has faced this issue or know about this ?