Just inspected one like this.

It was a concern, no one could figure out.

On going construction and the floor was dusty and dirty upon arrival. Using a toothbrush and minimal moisture, I was able to remove the majority.

Then under magnification, the white that was left was not soiling, but the bottom of the rough contours reflecting natural light at a different angle than the surface reflection. Like a mirror in the hole.

So, block the light and see if the white is still visually seen, from a standing position.

That sure looks like surface contaminants to me. Looks like my floor after the drywall guy finished up. He didn't believe in covering things. Not even a new floor.

I've also seen the grain fill on a dark mahogany shelf that I used paste wax on. Grain filled with white that wouldn't come off.

Try to clean a small area with a cleaner that works well on residue. I always recommend to get a spraybottle of TSP (ready mix spray which I often buy @ HomeDepot).

Mist an area and use a medium scrubbing brush (not too stiff/abrasive) and lightly scrub, wipe thoroughly clean with damp towels. Next neutralize the TSP eresidue with 1 part White vinegar / 9 parts water. Once dry monitor the effectiveness of this, if it works do the whole floor this way.

This is often the most effective way of removing residue, it may be that it is from oveeruse of floor cleaner, it appears the grain is fairly open and residual cleaner may appear whitish on such a dark floor.

Johannes.