Green line at the top left side of notch!

I bought the iphone x in the November 2017. After 3 months, that phone completely shut down and cannot do anything. So i go to the local store and about a month and a half, i got a new one ( this one). Now in the last three or four days, at the left top notch, the green line appearing and the screen becomes greenish color. But sometimes It disappeared. But my warranty has expired! I didn’t do anything, no water damage, no falling damage. What should i do? I think if i go to the store, they will charge me to repair because of lack of warranty.