Marco is on a first name basis with Star’s dad. When he first arrives in Mewni to bring Star the cereal (precious boy), he calls the king “River”. Yet later when he’s rescued by Buff Frog and Moon, he calls her “Queen Moon”.
Which means they’re relationship hasn’t developed enough for him to be comfortable calling her by her first name and vice versa.
What I’m saying is that at some point we’ll get to see some more quality development in Marco and Moon’s relationship and how that affects his relationship with Star.
Maybe not in episodes like the camping one and the one where River trashed the Diaz’s house, but maybe in an episode where Marco confronts Moon about Eclipsa and encourages her to open up more to Star.