I phone 6 screen turns gray on the top left hand corner

To make sure that this is not software related, set it back to factory settings, without using any backup data afterwards. Set up the rest of the personal settings manually and test the screen. If it still has the issue, this might be a hardware issue and the phone has to be serviced by Apple or an Authorized Apple Service Provider:

Use iTunes to restore your iOS device to factory settings - Apple Support

Find an Apple Authorized Service Provider

iPhone - Contact Support - Apple Support

Did you use any apps that used that area of the screen for a prolonged time to display static image?

Maybe you should visit an Apple Store or Authorized Apple Service Provider and show them your phone, just to be sure. Since your device is still under warranty, this should be for free.

Same problem just started with my iPhone 6. Did you find out the cause of the problem?