hello

[bezza]

So sorry , wrong photo! Still not too great at computers. With any luck this is it , with the fungus which is now gone.
The replies were useful anyway , as I didn't know the name of the one I posted accidentally.

[MikeT]

It's fungal, but definitely harmless. You could well get a fruiting body again later in the year. I think the fungus lives on organic matter such peat in the compost. Nothing to worry about.
I'm no Ferocactus expert but I think the picture looks plausible for the label (unless someone better qualified wants to say otherwise)