Could be either software or hardware problem... Did you had any fall damage? Is it quaite an old device? How did it appear for the first time? And first of all.. do u still have warranty?
How it happened for the first time? If it's after an update or smthing.. a reset could fix the issue... Otherwise try a green screen prevention app.. i don't have any recommendations but look up in google.. there are to prevent the issue but I've seen some people say that the app fixed their issue for a considerable amount . The other last thing you can do is I guess a display replacement which is not gonna be cheap.
