That's the jeweller's rouge the media is coated with. I'd throw it away and use the corn-cob.
I've never had a problem with it being left in the case but everything else got dirty and stained red (including my fingers) so I binned the stuff as soon as I had some replacement media with the green additive (Tuff-Nut?).
Then went to stainless steel pins for the majority of my case cleaning, the vibratory cleaner only gets a look-in once in a while so the media is lasting a long time.
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