If that grey band was on top and flickering, problems with touch... I would say you have touch disease, but from what it loos like in the photo looks like it is the screen itself or the LCD connection.
If you are experienced enough to open your Iphone, power it off, then remove the shield for the battery first...unplug the battery. Then remove the shield for the screen connections keeping careful track of where each screw goes (IT MATTERS)...unplug the screen and look for debris or corrosion, etc... then reconnect it...applying battery last and check it. If it is still there, I would guess its the screen.
Try steps 1 - 18 and then reverse to put it back on:
iPhone 6 Plus Front Panel Replacement