It seems that you are using a screen guard. I would recommend you remove the top portion and check whether it is the glue of the guard, or a problem with the display itself. If it isn't the glue, I recommend you turn your phone off, as this might be screen burn in, which happens on AMOLED phones. Leave it off for 30-45 minutes and check again to see if the issue still persists.
We apologize for the inconvenience caused. We would suggest you to visit nearby Samsung Service Centre so service engineer will examine your device physically. Please find service centre details in the link provided http://www.samsung.com/in/support/servicelocation/ Be sure to hit ♡ when you find an answer that works for you.
