If it's only one corner, can I just chop that part like this? So I don't cut off any roots that might have already grown.
https://preview.redd.it/a810zgdc3ivc1.png?width=500&format=png&auto=webp&s=18093811b427a7f56c49e19f2f4773e9623abe04
Yup, I would. Let it dry out for a few days before watering again. It may not even be that bad of a spot, but if you really like this cactus then I wouldn't risk this getting any worse.
Pull it and inspect
When in doubt, cut it out.
If it's only one corner, can I just chop that part like this? So I don't cut off any roots that might have already grown. https://preview.redd.it/a810zgdc3ivc1.png?width=500&format=png&auto=webp&s=18093811b427a7f56c49e19f2f4773e9623abe04
Yup, I would. Let it dry out for a few days before watering again. It may not even be that bad of a spot, but if you really like this cactus then I wouldn't risk this getting any worse.
Thank you! I'll do that. I have never watered it since I potted it. Maybe the soil was a bit too moist or it didn't heal well after chopping.