The white stuff is polishing compound.
It will probably scrub off with a pipecleaner dampened with a bit of alcohol.
If it does NOT, It's because the slot itself isn't polished. The bits of compound are EXTREMELY small, and any roughness/irregularity of the slot interior surface is enough to trap and hold them.
Polishing slots is simple enough with the right tools, but pretty much impossible without them.
The near-microscopic quantity of powder isn't anything to worry about in any event. It's just unsightly.