If it isn't mushy I wouldn't worry about it by the looks of things the stalk is just starting to harden and turn woody.. same as a young tree would as it gets older. The top of the cactus is a compleatly different cactus to the base that it is on ... if you are worried about the life of the yellow part of the cactus remove it and graft it to a desert cactus instead. It will help it live longer and gives it much better living conditions. The green part of your cactus is a compleatly different cactus all together and is actually a tropical cactus with different watering needs. It's like attaching a cat to a dog and feeding it fish food... neither will survive long unless you seperate them and give them the individual care that they need. both parts look compleatly healthy so I wouldn't panic, the stem is just coming into adulthood :)

That looks like a large terracotta pot. It might have a drainage hole. However, it looks too big and I can't see the kind of soil in it. I'm guessing that isn't a pot full of quartz sand. However, if it is a large pot full of quartz sand then I can see how that might be difficult to correctly water to keep the plants healthy. The two spiky plants in the back look like Hoodia. They are Asclepiads rather than Cacti. They look unbothered about anything. The front left plant might be a dark-leaf Aeonium. However, I'm not sure the stem should be that dark. The stem might be rotten. The center red ball cactus was grafted on to a root stock and the root stock died. Unfortunately the red ball cactus can't be saved without grafting it on to another root stock. The dead root stock is a common problem with that plant. I agree the P. afra in the back right and Echeveria in the front right look like they can be saved. You might have to cut the long stem off the Echeveria and reroot the top. If those were mine I'd prefer to plant each one in separate smaller pots. Group plantings in a pot bought at a store often don't work long term. In this case the plants might look like they need to be watered but have been watered too much, or been moist too long, and lost the healthy functioning of their roots. Without healthy roots plants can look like they need water even when the soil is moist. Some people asked if the dish was purchased "as is". I'm guessing they meant to ask if it was purchased as a group of plants in one pot rather than ask if it was sold marked as is meaning no guarantee the plants were healthy. If they were sold with no guarantee then I wouldn't bother trying to return them. Sometimes I buy plants like that.

Sooo... Looks like a Mammillaria. I'll wait for someone with more expertise to finish the ID. SOIL: This is very important. An easy (and good) soil mix to do is 60% perlite and 40% cactus and succulent. Easy to get ingredients. If you want to use a more hi-tech mix, you can search 'gritty mix' in the forum search bar above. I would stick with the simple mix for now. WATER: Only water when the soil is completely dry. You can check this with a wooden skewer or toothpick. Wait until the soil is completely dry, then wait a few more days after that, then you can water again. Pay close attention to see how the cactus responds. You may have to monitor the mix pretty closely for the first few waterings before you get the hang of it. POTTING: The pot size you have it in right now is good. Shoot for a pot with a drainage hole. Terracotta is good. SUNLIGHT: Mammillarias like a lot of sun. Put it outside if you can during the warmer months. Make sure you ease it into full sun so it doesn't burn. Pay close attention to it during this transition period. Indoor lighting over the winter is trickier, but you can cross that bridge when you come to it :) Also, prepare space for a lot of plants :)

The pot it’s in is a glazed pot with an attached tray base with holes allowing water to flow out from the soil. I don’t let water sit in there. I do have the pot sitting on top of another tray which I fill with water. There is no way (as far as i know??) for this water to get into the soil because the pot is glazed and there are no holes through the bottom (they’re on the sides inside the attached tray). I read that it’s a good way to give the plant some more humidity. Is that not true? I like the glazed pot because it seems to hold more moisture than unglazed but not as much as plastic. The way I check the soil is just by touching it a bit below the surface. Is there a better way you’d recommend? Any ideas why the soil would look so old and deflated after only a year? I mixed in a significant amount of perlite. However greenclaws up above mentioned it looks “peaty” and sure enough i just looked it up and the base mix I used is mostly peat moss and peat hummus . What do you want in the mix instead of peat? I will look for an African violet mix. I have never found a good potting mix for this plant, I hope i can get African violet mix here. I would love to make my own potting soil but have not had luck finding the separate components. I felt lucky for finding a bag of perlite! It’s all just pre-made bagged stuff. Even at greenhouses. thank you all for your input. I wish I knew what was the primary stressor on the plant. Was it the direct sunlight? Sounds like the cold isn’t a big deal, and it wasn’t looking great even before it got cold. Is it the potting soil being all used up or whatever is wrong with it? I can repot and then put back in the room where it gets good light but protect it behind a sheer curtain. Does that sound like a good plan?