It appears to me to be too wet plus something else. Perhaps a fungus or over fertilizing. What conditions are you growing this seedling in? Also would like to know what is the reason for all the white scale on the leaves?

I'm using 5 drops of Superthrive in a litre of water.
They're on a window sill with heating mats underneath.
I think the white stuff is just where the mist dried out. Maybe I'm in a hard water area.
Will let them dry out a bit.

Hopeful,
Remember, they are succulents, a little water goes a long way with plummies.
I buy distilled water for alot of my plants from Walmart, by the gallon. It's really cheap and the plants do better with it.
Check it out, or even at a pharmacy.

Thanks for that.
I usually make up a litre at the start of the week, use it for misting morning and night and then use the remainder as a good soak at the end of the week.
Am going to upgrade the soil and see how they react. And nipped off the two sick looking bits. Lol.