Here's the pun: "液" (ye) means "liquid", so you could translate it as "missing you juice". "夜" (ye) means night, which is why Mokou is looking longingly at the moon.
Somebody please do this amazing pun justice
Updated
I-miss-you intravenous dripA liquid (night) where I think of you Lit. "the I-miss-you liquid" or "I miss your juice" depending on where you put the accent. "Liquid" and "night" sound the same (ye) in Chinese.