Racism exist because of our greedy and mean ancestors who only cares for themselves, black people commonly treated as a slave before , nowadays those humiliations people still infected by the idea about hierarchy.

We are the only humans here on earth that have segregation of race, i'm not a black, i'm not an Asian, i'm not a Latino-- "I'm a human being".
The aliens don't call themselves a big bald head, they don't call themselves a lizard, or three eyed or any other creepy species, they call themselves " Alien ".