Just ignore it. From what you wrote, I see you two are not in contact, and thinking about him doesn’t seem to make your life troblesome. If it doesn’t actually affect your life, just let it be and move on with whatever you’re doing. If it doesn’t create damage, there’s no need to fight it. I feel you in a way. There’s this person that is sitting in my head for two years now. We never had anything, but he keeps popping up in my head from time to time. I got used to it eventually and focused on other things.