Potting soil should be really a potting mix. IMO, you should add lots of sifted (to get rid of dust and smallest particles) perlite or pumice, like 1:1 ratio. Let mix dry up between watering. Don't overfertilize, and only during growth period. Put in a sunny spot. Make sure there is good air circulation; if needed, use a fan. Prune off any dead blooms and leaves.

Thank you! Any idea about the white powdery film?

I am not sure about exact treatment, but good air circulation helps. Moist, warm and stagnant environment will encourage few fungal diseases. There are few fungicides available to treat them. I do not know what to use - I have kalanchoe plant for quite a few years, but didn't have same problem.

Depending on condition of your plant (can you post a photo of the whole plant?), maybe it would be better to prune it, use well draining mix and increase air circulation. I rejuvenate mine regularly, since they eventually need it IMO.

Thank you for your advice!

I've added more perlite to the soil. I hope these pictures are helpful. They're pretty spindly anymore. It's hard to get natural light in my apartment and sometimes the windows are too cold so I have to pull them back. Not sure what's worse, cold or less light. Right now the most troubling thing is that leaves are losing their sturdiness, but the soil is moist for about a week and a half, so I doubt that under watering is the culprit.

Thank you so much for the tips. I'll trim those and monitor the wetness of the soil. It's not sopping very wet for a week and half, but it still feels moist about an inch or so down. They have holes to drain.

That is probably powdery mildew (a bane of begonias). I agree with rina, keep it warm and provide more light (LED has become very affordable and is very bright and uses the least amount of power). Keep Kalanchoe more on the dry side to discourage mildew outbreaks.

For sprays, you will find a smorgasbord of things to use on this site alone. I tried Neem oil (for begonias) but the smell is pungent - some like it while others don't. Milstop which can be bought online but is kind of expensive since you need to buy a bucket of it (five pounds I think). I switched to Orchard Spray which is liquid sulfur and pyrethrin. I usually find one to two sprays over the course of a winter stops mildew. Sulfur has its own smell but quickly disappears unlike Neem which may take days to disappear and may need multiple applications through the winter months.

As the ballasts burn out on my T-8 shop lights I am replacing them with LED shop lights versus changing out the ballast (a simple job and about $10 per fixture). The prices of LED just a few years ago was prohibitive (60 to 80 dollars as I recall) while T-8 was around $26 (cheaper sets from Home Depot) and I've included cheap tubes in the price. The latest LED lights were $20 at Sam's Club and are twice the light output going by the free light meter I installed on my phone. The energy consumed is a lot lower as well.

Sorry I don't have any recent photos of Kalanchoe but here is an example of Earth Stars under fluorescent lights in another room of the basement.

Powdery mildew is a possibility, also result of higher ambient humidity and poor air circulation. These are cultural reasons (same as for oedema), and should be fixed somehow easy. I have same kalanchoe for years, and do not get it. But I regularly rejuvenate plants, do not crowd them, keep them in sufficient light and do not overwater - I water well, but fast draining mix they are potted in doesn't stay wet/moist for prolonged periods of time. This applies to al succulents. I never used any chemicals to spray, but they could be used if needed. Surprisingly, spores of powdery mildew could be killed by water, but do not drench leaves unless you are very sure - water will encourage many other fungi. Your pot(s) have drainage holes as you said (good!), but soil is probably water retaining. It is best to amend it with lots of sifted perlite or pumice, so it drains well.

Your plant could look much better in few months if you follow tips suggested...sturdy stems, bigger leaves, richer green color, much more compact...(those dark pieces are drying-up, spent blooms, which get cut off):