Some of us do have trouble telling people apart and we are not racists. I'm in customer service and I have a hard time. One blonde white woman from another? Not easy. One Shaven-headed square-jawed black man from another? Not easy. Yeah, I can't always tell black people apart, but it's also white people, latino, etc.
Happens to everyone, I suppose. Whilst living in South Africa, I had little enough to do with non-whites, but the blacks of the ethnic group predominant in Joburg had a tendency to wear knitted hats on a daily basis, making it hard for me to tell them apart, as the mind lumps people you don't know and who kind of resemble each other together, for economy purposes.
On the other hand, there's a certain type of young, slim, white blond woman my mind insists look all the same.
Doesn't make us racist. Makes our brains effective engines, though. Although in my case, I have trouble remembering faces anyhow. No photographic memory to speak of, sadly.
Sheds a different light on all those times people confused me for my sister....
One has to keep in mind that aliens aren't all analogous to different human races on Earth. In Star Trek some aliens are smarter, some are dumber, some are greedy, some are warlike to name a few stereotypical traits.