Simple! Get rid of the tuff-nut media! No kidding, it's junk. The red stuff is rouge, finely ground iron oxide,(rust)! It has a bit of oil mixed with it, that's the film you see.
I wouldn't want an abrasive left inside my cases. Even a mold abrasive like that rouge.
Get this extra fine corncob media from drillspot, shipped free. It's fine enough as to NOT plug flash holes if you elect to tumble after decapping.
http://www.drillspot.com/products/52...bs_blast_media
If you want a high shine as well as clean film-free brass, a dab of car paint scratch remover added to the corncob works well.