Hello!
This morning I noticed a strange crust spreading over my cactus plant. Is this scale or something else?
If it is scale, how can I save the plant?
Thank you,
Aimee

Its not insects or disease. Something is not quite right with the root system so the bottom of your plant is drying.

I would suggest to take one of the healthy pads from the top and start your Opuntia mycrodasys
over again. That may be a sign of root rot.

I am thinking the media has somehow stayed too wet, since it does not look gritty enough and then being by the window it got cold, so the roots may have suffered from some rotting.

Observe a bit more, it may just try to dry up some more, and just resume growing from there. Give it as much warmth and bright light as you can.