I'm not sure what type of cactus I have, but I was told when I purchased it, that it was 30 years old. I've had it for just over a year, and it's done great until now. I think she's overwatered, but would really appreciate any insight whatsoever. I've taken these two pictures, and can capture a broader view if that would be more helpful.
i'd say it's a euphorbia rather than a cactus. except for that one piece, it looks okay. i'd recommend getting that bad piece out asap - if it's got something it could spread to the others and if you just overwatered, you'd be able to remove some of the overly moist soil as well as check the roots of the others and remove any mushy bits before the damage travels up. as for the piece that's going - i do see some healthy bits on it. just cut off above where the rot is and let it sit for a week or two to callous over. be sure to put the piece over some kind of holder as euphorbia drip sap - it's very sticky and it can be an irritant, so you'll want to catch it in something so it can be disposed of in a manner where no one (human or animal) will be hurt by it. let the cut piece sit for at least a week and then insert into a mix of cactus soil. don't water it right away though! let it sit for a couple of weeks in the fresh soil and then give it a drink. as for the main pieces/pot - if you did overwater it, i'd definitely pull it out of the pot and let the roots and soil dry out for a bit. spread the soil out on a half-dozen layers of newspaper and you could just put the plant back into the empty pot until the soil and roots have dried out a bit. then get it potted up again. it's a lovely specimen and i'm sure it IS 30 year old!! would be a real shame to lose it now!! the pot that it's in - does it have drainage holes? if not, repot it in something that does! they don't need a lot of water and the soil should dry out very well before rewatering. unglazed clay works best for these guys!
