First a disclaimer..this is about GAME racism. I was trying to think of things that each race would call others as a way of insulting them. Here are a few, feel free to add as you see fit

Elves = twigs, find them in the forest and they snap sooo easily, burn well too

Dwarves = furfaces, furballs, flea catchers, flea traps,

Halflings = Shorties, bugs, (because you can step on them)

Kolbolds or other reptilian races call humaniods smoothskins, or scaleless

Orcs call others toothless or tuskless

and its so much fun as a DM to tell a player running the elf that the orcish insult he heard translates to "hey twig your mommas does flea catchers"