Very fine powder on base of two of my orchids

[meliana]

I just noticed something like a fine, off-white/ light brown powder on a phal and oncidium. It wipes off easily with my finger, and I tried to get a good picture of what was left. It's concentrated on the base and the tops of the roots, and so far is only appearing on one side. I repotted the phal a few days ago and didn't notice it before that, could it have hitched a ride on my potting mix? The oncidium is the next closest plant.

I couldn't seem to find reference to anything like this, does anyone have any idea what this could be and how to treat it? My resources right now are unfortunately limited to original listerine.