Aura Mutation vs. indestructible enchantments
The spell still resolves and gives you tokens; being indestructible means nothing more than


  702.12b A permanent with indestructible can’t be destroyed. Such permanents aren’t destroyed by lethal damage, and they ignore the state-based action that checks for lethal damage (see rule 704.5g).


(source: Comprehensive Rules)

This is different from being an illegal target because of protection from white, shroud or hexproof.