After writing that song with Simon (or "Ice King" as he now refers to himself as) Marceline was emotionally exhausted. Sure she was mad that Ice King didn't remember her. But she was also mad at the old jerk for not even trying to remember her or what they had done together. But after all the times they sang the song, she was also physically tired, so she decided to take a nap.
Sadly for Marceline, she was stopped right before she could get to her room by the most annoying knocking at her door. She expected o see Ice King saying something like "Marceline I need a new song! The princesses still don't wanna marry me!". But instead she was screamed at by a worried looking Finn who wouldn't stop yelling, "MARCELINE ARE YOU OKAY!"
She simply responded, "What do you mean am I okay?"
"NO ONE JUST LETS ICE KING IN THEIR HOUSE LIKE THAT" Then he just started shaking her back and forth.
That earned him a tackle from Marceline. "STOP. ACTING. CRAZY. Glob you're acting crazier than Simon. And he was totally NUTS man!"
"Oh, sorry Marceline. It's just I'm not used to seeing people just let Ice King into their house like that. It's just weird."
"Yeah, well we were friends a while ago."
Finns jaw suddenly dropped.
"Friends with ICE KING? You're sure it wasn't someone else?"
"Yeah, come in and lemme explain. Just promise not to act all crazy and junk."
"Whatevs"
She spent the next few hours telling Finn about all the stuff she and Simon did, back when he was still Simon. She told him how Ice King found her crying in a street when she was a little girl, how he gave her Hambo, everything, By the end she and finn were both tired. Marceline emotionally, and Finn physically. So eventually Finn went home to sleep, and Marcie began strumming at her base and singing the song she sang with Ice King earlier that day, and fell asleep in the middle of the refrain.