Random Lander headcanon of mine: I imagine Luke being self-conscious about his eyes. Like, I can see him thinking they're too bright or he just isn't a fan of the color. So, in return, Zander likes to call Luke "my insecure Lemon". Which eventually turned into Zander's pet name for Luke "Lemon".
Idk, I just thought it was cute and wanted to share it with you guys.