White dust coming from dash AC vents

In our 97 Intruder it might have been from when the previous owner set off a fire extinguisher, just before it was repossessed by the lender.

You won't know what the dust is unless you send it to a lab to analyze it. It is whatever is in the air when you are moving. Your ventilation system almost certainly draws air from outside, and probably does not have an environmental air filter.

I've had three vehicles now with the filters, supposed to be sized to trap pollen (which comes in a wide range of particle sizes) and I've found they get as dusty inside, as fast, as those without filters. Where I am, a lot of what gets through is smoke particles, much finer than most of the pollen filters are supposed to trap.