I only have a 40X jewelers loupe to examine my cartridge. When this Onzow issue first was reported, I noticed similar looking 'dots' of whiteish grey spots on the cantilever and cartridge body magnets, while gently trying to scrape them off, I noticed how strong the magnetic attraction was on the bottom of the cart body (Audio technica ART9), Dunno if the Onzow debris is ferrous, something in my environment dust (did have kitchen remodeled recently) but for the last month have been using a photography lens brush mainly with the Onzow every 10 days or so and no longer see these whitish grey dots.
I agree, there's more to the story and just wanted to inject the magnetic part to the considering, my VPI prime does not have a dust cover so that's part of my environmental dust concerns. Gonna turn my hepa air filter back on.