Six just sees it as a way to eliminate one more variable in his everyday life. Also, he likes green.

"He's happy I made it instead of, y'know, not making it here." Rex gets what the guy's trying to say. "Still a little weirded out seeing him in a kitchen or a dining room instead of a lab, but I'm getting used to it. To his roommate and fiancée, too. Didn't realize how much César likes coffee until now!"

He's trying to feel out how their relationship has change and how best to be a little brother and dump on his brother. Gotta figure out how to tease him. Rex doesn't remember César from when he was younger, but he knows he's supposed to make fun of his brother a little. Lovingly.