Harrie, usually when mesembs like your Pleiospilo are regenerating its leaves like the one on the left (notice how new shoots emerges from the center?), never to ever water them until they completely absorb the outer leaves until it dries up.

I trust the mix is probably too wet, and it's probably turned to rotting mush. A picture of how it looks like will help.

And also state your region/zone. Mesembs do not enjoy really hot summers or extreme cold temp.

It sounds like you are growing them indoors, just ensure they get as much light as they can. Never put them in the dark.

Bernard

This will be helpful too:

http://www.cactus-art.biz/schede/PLEIOSPILOS/Pleiospilos_nelii/Pleiospilos_nelii/Pleiospilos_nelii.htm

Harrie

Is there a drainage hole in the shoe planter?

uh oh. same thing happened to my p. nelii. you must move fast if you want it to live. What is happening is rot. you should cut off the rotting parts with a clean knife. and let it dry and callous over.

it may live if you cut off the rotting parts fast enough.

do not water at all. what ever you do, dont water.
I suspect, as rina probably does too, that the shoe planter doesnt have enough drainage to keep the soil dry for succulents.

Did you water right after you planted it. That is a huge no-no. One always waits atlas a week for any succulent.