It looks like sunburn, especially if the plant is leaning (or accordioning) so that its growing point faces the sun. Softness doesn't always mean rot, especially for thin-skinned and soft-bodied cacti. Mammillarias as a whole are rather good at shrinking down and clamming up, especially the ones with mini-parasols on each areole. Here's another example. It's a Stenocactus that was sitting in a dark area for a while before I bought it. Not quite the same thing, but you can see the yellowed areas and the plant as a wholes's reaction to the increased sun. http://imgur.com/a/J7GM7 Pictures are in the order front, back, side, top. 'Front' is the side that gets the most sun, which in the UK doesn't get very high in the sky. The very bottom of the plant being covered in those spines helps with the sunburn problem. There's a Ferocactus seedling I bought whose growth before I bought it was turning an alarming shade of faded brown even in the late April sun. So I slotted its pot into one slightly bigger and used the increased height to lay down a thick gravel layer. Removing the damage is only for cases of rot or other infections that will spread faster than the plant can seal off the affected areas. Watery-squishy is bad, rubbery-squishy is less than ideal but nothing that the plant can't handle with either time or TLC. The difference is a little hard to explain, but soft tissue resists more the harder you press and rotted tissue doesn't so much. You can cut a small slit in the skin and press around the area if you really want to make sure, as tough skin can put up its own resistance even if what's underneath is going mushy. The damage won't go away, but it'll eventually be folded into the base of the plant and disappear from view. Or maybe the plant will give up on the whole area and in several years time you'll find a patch of tubercles peeling off and not remember why. Here is a link that might be useful: Stenocactus pics, clickable URL

I will identify them in order: 1. Some Sansevieras in back with Kalanchoe luciae in front. 2. Crassula ovata 'Skinny Fingers'. Way underwatered. 3. Aloe aristata. Looks okay... For now... Also in that picture is a (Mammillaria?) cactus and upper right will need a closer picture. 4. Aloe... Unknown species, but it is in poor condition and may not recover... For most of these, the diagnosis is easy: It is a soil problem. However, it is pretty easy to fix. You will want to amend your soil plenty of perlite. Close to 60% perlite and 40% cactus and succulent soil will work. The Crassula ovata definitely needs to be watered. It should survive, owing to the thick stems... I'm not sure if the sick Aloe will make it. When you repot, feel around for mushy or smelly roots or stem. Anything mushy needs to go.

Yeah a bit better, thanks. I can't see the bottom of the stem at all, but that might be coz it's in shadow. There are some leaves going brown, and a red shriveled up one that's red-ish. I can see (or think I can) that you've moved some of the top stones, but I can't see the mix enough to tell if it's a good mix for succulents. Plants will loose their bottom leaves naturally anyway, I just want to rule out rot as a possible problem. If you can provide as much information as you can on the below questions marked with a # we can hopefully tell you the plant is ok and how to look after it. If it's got rot, we can provide advice on how to save it. # Could you feel those leaves and tell me if the leaves are are soft and squishy. # Can you see the base of the stem and tell me it's colour, eg is it green like the upper stem, or black. Also feel the base for squishyness. # C you move the top dressing stones away from an edge of the pot and scrape back the thin surface part. Then either .......1. take a photo of the mix it's potted in, but with good light on it. ......Or 2. take out about a teaspoon of the mix, put it on some paper and take a photo so I can see what sort of mix it's potted in. # Was it wet or dry when you bought it. # How often do you water. Just to explain a bit. Succulents (which include cactii), when watered, need the water to flow through and out, so it draws the air down to the roots. This provides the roots with needed oxygen. Then it all needs to dry out completely before being watered again. Therefore soil that retains water for a long time isn't good. If it takes too long to dry out, at some stage it can start to rot. The rot then will travel slowly up the roots to the stem, leaves and eventually kill the plant. Often people don't realise that all the roots have died until the bottom stem and leaves start to rot. If rot is suspected it will need to be taken out of the pot and inspected. If it has rot in some of the roots they can be trimmed off. If it's got to the bottom stem and leaves the plant can usually be saved. Higher up is more dicey depending on how far it's gone up, and the type of plant Sorry that this is so long. Hopefully someone more experienced will jump in to help, and fix anything that needs amending.

Thank you all very much. I returned to work on 'the mass' today, here it is before I started: I had to soak the roots to remove the stuff, and it was still pretty painstaking. Without the water the roots just broke off with the peat, which was pretty rock-hard. Perhaps this means the roots are in a bad way? It took a long time, I had to remove it fragment by fragment even with the soaking, I did not like to soak too long. Here it is now: Looks like a little hairy spiny monster here, very undignified, upside-down, drying off: I think there are still little peaty fragments around the roots that I ran out of time to remove today. Just how thoroughly do I need to clean them off? I am concerned the darkness around the base might be rot or the beginning of it. It certainly has some 'give' though i'm not sure there's anymore squishiness than the other parts of the cactus, because I can't feel them with my finger in the same way because of the spines. If it is rot, is there anything I can actually do about it, aside from keeping it dry? I did get rid of some of the crap stuck in the spines after this, though there's some more left to remove. Any tips for getting stuff out of cactus spines? I used a stiff paint brush. Funny, I was given one of those silly cacti glued into it's pot and that was actually easier to deal with than this. I really understand why some people hate peat so much now, assuming that is what it was, of course. Maybe it was actually originally planted in some unholy mixture of dirt, perlite and glue : / Thanks again!