iPhone 6 screen burn
If you have an Apple Store close to you definitely go in and/or book an appointment with a Genius. They will be able to take a look at it and advise you what has caused it. If it is hardware failure and your phone is within warranty, they will generally replace the screen for you on the spot for no charge. More often than not even if you are out of warranty, they will still replace it for free or for a much reduced price.

I agree with the others that this is a hardware issue with the LCD panel.