Anti Starco is the paring of Star and Marco as rivals or enemies that's never going to happen ;P
Canon Probability Edit
Although Star and Marco have opposed each other many times, the pair becoming enemies or rivals is highly unlikely considering their strong bond of friendship (and romance). That does not mean that the ship will never happen.
Daron Nefcy (Aka the show creator) originally was going have Star and Marco be enemies, and both being in elementary school.