This is a Euphorbia, a crested E. lactea grafted onto E. neriifolia, not a cactus. Does not require misting or a humidity tray. You cannot fix the damage that has been done, only prevent it in future. Check for soft spots, which would indicate rot. Does the pot have a drainage hole? This is a necessity. A rule I follow about watering is, "When in Doubt, DON'T." More houseplants die from overwatering than any other cause. Christopher

You have a jade, aka, crassula ovata. It is in desparate need of light. Was this where it was when you moved in? If so, do you have a balcony on which you could slowly acclamate it when the outside temps are above 55 or so consistently. If you just moved it to this spot and it is better-lit than where it was, these could be sunburn marks. Typically sunburn marks are more uniform in shape, though. It should have about 1/5 the amount of space it has between the leaves. It almost looks like it is supposed to be a Crosby's Compact which should have all the leaves very close together with proper lighting. Jade grows spindly when it isn't getting enough light. I would take it out of the plastic pot and repot it in a clay pot. Buy one that is shorter. THey are sold as azalea pots. Your soil should be very free draining. When I mix mine, I use one part pine bark fines or soil conditioner, one part grit (decomposing granite), one part high fired clay granules (turface, aquatic soil), and one part small pea gravel. When you water it, water should run almost right through it. If you use peat based soils, they will compact and stay too wet, thus strangling the plant due to lack of oxygen. Even the soils that say they are for Cacti and Succulents are too peaty. You could also try coir inplace of the pine bark fines or in addition to them in place of peat. Almost anything you would use for a bonsai would be okay for a jade since they are very free draining. They don't need a lot of fertilizer. Use something low in nitrogen. I use Schultz Cactus Fertilizer mixed for every watering, but I only give it to them every other watering during their growing season. It is warm where I live until Oct or Nov, which is usually when I stop feeding and slow down watering to once a month until it warms up again in spring. We were warming up in March and then had a frigid spell over Easter holiday. I've gone back and forth with my plants, but having a mild spring has been nice for a change. I hope this helps, and don't hesitate to ask more questions. Do searches on this forum about Jades, and you'll get more than you ever wanted to know about jades. They are, without a doubt, my favorite. I have about 10 different plants, 10 starts from cuttings and two that are bonsaied.

Im late but this just happened to mine and this is the only post I could find that looked like it, except mine only had the white mold. Yesterday I did my routine poke check to make sure everything's good and It was perfectly fine, today I walk past it and noticed it had mould all in the cracks of it, I pushed a spine and it cracked open and was full of mush. Cant figure out what caused it either, had it for years and it survived a mealy bug attack and everything. Now I'm on a mission to find another one...

Thanks Kara for the quick answer. Yes it's the bright white fuzz! They were making me crazy but now they're almost gone (I think). They are in a cactus mix I bought online. It says on the website it has standard potting soil, washed sand, coconut shell powder, worm castings, crushed pine bark, perlite and crushed vegetable charcoal. I have two other cactus and some succulents in the same mix that are doing great, no bugs or anything. I keep them far from the infected ones. Is it ok to repot again since I did it just 3 to 4 weeks ago? Ew, scales. I'll prepare for surgery. There are some small orange spots between the big black ones, could them be scales too? Just in case, I took more photos and zoomed in on the light brown spots of the cactus of the 3rd photo: Are they scars or scales or both? Thanks!