It's not racism. Seadwellers and landdwellers have a sort of rivalry. See: Vriska and Eridan.

CG: YOU KISS THE GROUND THIS LUNATIC WALKS ON BECAUSE HE HAS PURPLE BLOOD
CG: BUT THAT DOESN’T STOP YOU FROM RIPPING ON ERIDAN, I KNOW FOR A FACT YOU DON’T LIKE HIM
CG: AND HIS BLOOD IS EVEN PURPLIER, ISN’T IT?
CT: D –> Yes
CT: D –> That’s different
CT: D –> He is a sea dweller
CT: D –> Our feud is codified in tradition
CT: D –> Neigh, we are obligated to be at odds
CT: D –> It’s dignified

Call it a rivalry if you want, but I mean. With Equius’ Casteism being such a part of his character, the biased demeaning of Seadwellers based on arbitrary traditional reasons, regardless of their actual status, sounds a bit more shitty than just rivalry.