I doubt that Black Walnut would spalt in a couple of weeks, and I will have to see the first piece of Black Walnut that has spalting in the heartwood, if there is any, it is in the sapwood only.
As for the mold spores they are around everywhere, you don’t have to be afraid of them, you are inhaling them as we speak, and yes they do get sucked up by your dust collector, if the dust is wet at more than 20%, I doubt that very much, as a dust particle would dry almost instantly going through the air, than mold could start growing in your dust collector, never seen or heard of that though