Hi & Welcome to Gardenweb! It's a Pleiopsis, the one at the bottom of the pic. That's what mine looked like, just before it turned into green pudding. It may be a goner.

Those guys are notoriously touchy about too much water, and often by the time it displays the signs of rot, it's too late.

Your soil looks to be too sandy, and completely soaked to boot. You can get perlite to mix in there, so the water runs right out of the bottom, rather than staying in that sand. There's a lot to read here about media ... but the takeaway for cacti and succulents is that sand = bad. You need a pot with a hole in the bottom and soil that's free-draining and dries out quickly.