Certified GKH Gardening Expert
Uh-oh... This does appear to be a fungal infection. You will want to apply wettable sulfur the next time you water, and cut off any infected parts. Rub the spot with peroxide to make sure it kills the infection from the cut.
Make sure to only water when the soil is almost completely dry. You may even want to use a cactus mix type soil, if you aren't already.