Unless the soil is less than 2 inches deep, yes, it's probably overpotted by a LONG long long ways, thus doesn't dry out fast enough.

I can't tell what the soil mix is either....hope its' well draining.

I agree, they like to be very very very root-bound (crowded in their pots) so I'd also suggest moving it to a smaller, unglazed pot filled with cactus/succulent mix (I use 1 part potting mix to 1 part pumice or perlite). They really don't like the summer heat in Tucson (and they hate the summer heat in Phoenix) so they often melt/rot due to the heat in addition to too much vacant soil, so very light on the watering, especially in summer. I'm sure you can save it though, so keep us posted. Good luck!