I gave up on nutshell, particularly walnut.
I was moving and decided to mass clean all my brass before the move and didn't really look at the brass. Found that all most all the cases had a very thick layer of very fine walnut powder that had to be individually scrapper out.
I would only use nutshell for cleaning really dirty or corroded cases -- and, at my age, I am a lot more willing to just not pick them up or throw them away.
I use 20/40 corn with deprimed cases. Works best for me.
You can try washing the cases (soak in hot water and Dawn, put in nylon mesh bag and toss in clothes washer, or put in basket and wash in dish washer) or, if it is no more than a film, simply ignore it. It does get other things, like your hands, all brown/red.
From now on, put some paper toweling or used dryer sheets in the tumbler to pick up a lot of the dust.