First it’s not a cactus, but a Euphorbia. They look alike but very different, good ol’ convergent evolution🙂.

It’s not rot, but corking. You inherited it, so I’m guessing it’s in a different environment than before. The Euphorbia could be corking from receiving more sun then it once was. What sort of environment was it in before compared to now?

Thanks so much for your response! And great to know its not a cactus. I say “inherited” loosely, but really I bought the house and the owners just left it here, so it actually hasn’t moved an inch.

Glad to know its not rot though. do you think its getting too much sun?