u/Freaks-Weirdos

So, I was fortunate enough to get a discount on these monkey tails, as I believed they had scale... But on closer inspection, these cream patches don't look like scale at all. Anyone got any clues?

Diagnostic notes:

1. The patches were found on 9 of the 11 cacti, and on a Rebutia nearby.
2. The patches vary from tiny dots less than 1mm to patches greater than 15mm.
3. The larger patches can be removed with tweezers, however they do take some convincing, and are well adhered to the epidermis of the cacti. see the last photo vs the second last photo in which i have removed the some of the patch, but the epidermis is left very glossy, perhaps having lost a thin layer off of the top.
4. The cacti were being displayed in close proximity, near a window, but without a lot of light. They had been there for quite some time.
5. There were several echeveria nearby with powdery mildew.
6. The roots and soil all seem health, although they were in very small 55mm pots.