I'm no C&S expert but to me the 'soli' appears to be very, very wet. It needs a porous and fast draining medium to thrive. Miracle grow C&S soil is not recommended by many growers on here even for use with C&S's. What type of mix was it growing in originally? This looks like a recipe for root rot and from what you have said that may be happening already.
If it were mine, I would un-pot, check the roots/plant bases for rot, cut away damaged portions, dry it out and re-pot in a fast draining gritty mix (and by that I don't mean sandy, the particles are way to small) I think your mix is far too fine and is wet and has compacted too much. I would also never water overhead, a light mist occasionally, but not drenching. Just my thoughts, I'm sure someone else with more C&S experience will offer more detailed info. Good luck with your little plant.

Gill UK.

Are they "mushy soft" on the edges and tops somewhat wiggly ?

Personal prefferenace to your pot choice I would have set it in a pot less bigger by it's square shape and a little deeper
Thechnicly in my opinion it is slghtly over potted but works

Soil concern ? Each chooses there own as to what shoud be used but all agree that good ( fast) draining potting materails are important.

My soil for these is pumice, gravel with an equal amount of perlite sifted with some somewhat less powdery from the very bag of C&S you may have found.
Not certain you would repot right away during the active growin season of now but with a problem like this I would certainly hope you do with high hopes for the better outcome.

Thanks so much for your feedback! I know the medium seems very fine, and very wet. That's because I put a dressing of sand on top of the potting soil, and had JUST watered it when I took the photo. It's completely dry now.

The funny thing is, a day after being deeply watered, the plant is much more springy/less wiggly. Is there a chance I just wasn't watering enough?

It sounds like when I repot I should be sure to mix more gravel and pumice in to improve drainage, but maybe I need to step up my watering, too, considering the intensity of heat and light where it's located.

Thoughts?

i had a similar problem with my aloe vera, there was a soft spot near the base of my leaves from sitting in the soil when i bought it back from the store (they planted part of the leaves in the soil), thinking there was too much moisture, i let it dry out near the window for a week and it got worse but it stiffened up right after i watered it. I hope that helps!

Just an FYI on your sand top dressing. I did the same with a small aloe. Over time, the sand worked its way into the soil mix (also C&S at that time!) and caused too much moisture retention. It turned into a mix of sand and soil. I was surprised that the sand "moved." OTOH - I am in a moist area with rainy summers and my plants are outside, so my soil mix was bad for my area to begin with.