Very moldy cast iron cookware - what's the best way to clean it?
The most efficient method would be to skip the whole bleach step, and start right in on burning the mold (and seasoning) off either in a fire or in an oven on the self-clean cycle. Then proceed to re-seasoning.

Taking the bleach side trip adds the joy of rust removal, which you can avoid if you don't let the cleaned (by fire/heat) cast iron get damp before oiling it and starting the re-seasoning process.