Iphone 6S Plus Transparent box on display

I have transparent rectangular box at the bottom of my display. Sometimes it becomes blue lines or even other colours. The display still works perfectly normally though. It typically goes in and out from transparent to normal and back again. Sometimes if the display stays on for long enough it becomes perfectly normal again.

A blue box also appears during boot up. I am not sure what the issue is or how if it is a serious issue needing a new screen. Can someone help me out.