https://gardenswhisper.com/black-spots-on-succulents/
might help.Or PM Hywel - sometimes still looks at goy.

Thank you so much Stera.

It doesn' t look very well though does it? I'd wait another month and then dig it up and have a look at the roots just to check for root mealy bug.. You wouldn't want it to infect that nice cristate cactus next to it.

Stera you are so right...the Cristate cactus is attractive.Will wait a little.