Doesn't sound good. Do you think the roots are healthy? I've been trying hydrogen peroxide to clean up sick plants lately and haven't noticed any ill effects.

Does this look like rot? I'm not sure why it would be I use cactus soil mixed with perlite then add more perlite to make it more than 50%. I water every week or two depending on whether the soil is dry. I'm worried this is one of my favorite plants.

A little bit but it's hard to say. I bet it'll be okay.