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.
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.
