When Relentless Skaabs returns to the battlefield because of its undying ability, it’s not being cast. You won’t exile a creature card from your graveyard.

You must exile exactly one creature card from your graveyard to cast this spell; you cannot cast it without exiling a creature card, and you cannot exile additional creature cards.

Players can only respond once this spell has been cast and all its costs have been paid. No one can try to otherwise remove the creature card you exiled in order to prevent you from casting this spell.