If you can see it on a screenshot, it's not burn in, like others mentioned. If you only see it on your phone, then it could be a few things:
1) If it is burn-in, you'd see the same 'ghost' image on anything you display on the phone, and if you rotate whatever is being displayed, the 'ghost' wouldn't rotate and would stay always the same on the screen.
2) Faulty display. From what you're describing, seeing half the screen with a different brightness or tone, might mean your display panel is faulty (either the panel is broken, the connector is loose, or just damaged). If whatever image/app you display on your phone changes brightness/tone in that half of the screen, and furthermore, if it seems to 'flicker', then it's pretty much hardware-related and you might need to replace your screen.
3) An app causing issues. VERY unlikely for display issues, but it could be an app making things act wonky. If this were the case, you wouldn't see the issue while booting up the phone, in Recovery Mode, or if booting into Safe Mode.
Like others have suggested, it would help to see an actual picture of your phone's screen, not just a screenshot
