Some play on an existing word maybe, delicate, feeble, frail, shallow, forceless. I had a similar problem coming up for a name for a group of people in a sci-fi story, although it wasn't a slur. I usually go to the thesaurus, on or offline and look up words that might fit. Also they don't always have to be directly accurate, maybe just subtly so.
Just call them 'Ancients' or 'Ancestors' and use a clever trick to turn it into 'the Anni' or the 'Cien' or something like that. It doesn't have to be a recognizable English word, and most slurs are typically an alteration. If the difference is physical strength or 'powers' then something which used to be an 'honor' could be warped into an insult. A simple system would be that the 'true humans' were the common ancestor, but their time in the sun has passed, an old honorific becomes a curse and identifying tag so that even recently borne 'Cien' are seen as inferior. For society purposes Cien could be marginalized by lack of ability.
If they don't think too highly of humans what about putting them on the level of an animal. You could call them 'Porkers' or 'Meatbags'. Stains or Stainers could work as well.
Here are some quick ones! Peasantbloods Waterbags Slaveminds Glass-spines Dirtskins Filfthbags Sun-squatters Greedhunters Mindfools Manatics Uhhh... Wormbreeders ... Ilkminds ... Nopes no more...
