When Rick had a fight with Pete, in that fight he got a scar on his face when both of them during the fight broke through a window.
Step-by-step explanation:
In season 5 B, a fight has been shown between Pete and Rick. In that fight, both of them broke through the window.
Because of this, Rick got a scar on his face. Later in a scene when Rick is seen talking to Carol, bandages can be seen on that spot which occurred at the time when he had a fight.