You were the only light of his days... well, other than his daughter Charlie. He'd never admit that though. He would just try to get along with everyone else at the Hazbin Hotel, both because his daughter told him to and because he didn't want his secret adoration for you to be obvious. He still talked to you, but usually when you two were alone... which wasn't often, but enough to make you two well-acquainted to each other.
One day, he just couldn't handle it anymore. He wanted to talk to you more, and he had this silly greeting he wanted to try. A play on words, you can say it was. It was originally meant for Lilith, but she left him before he could use it.
At this point it seemed like it was more meant for you anyway. To him, you were his little star. He also kind of wanted you to take his last name, and he thought it was a clever way of foreshadowing that.
"Morning, star." He finally built the courage to say the words after he saw you exit your hotel room one morning. He was fiddling with the apple on his cane in the lobby, secretly hoping that you got the lil' joke. He said it in a more playful way so that his love for you still wasn't completely obvious.