You can see information on this fungus here:
http://stalikez.info/fsm/semp/...
This fungus live
naturally with Sempervivum in the nature. If you have this fungus in your wild rosettes , you must delete these plants to avoid the propagation in your collection.
This fungus is not very bad and easy to eradicate
I often get wild rosettes (or I collect ) and I only encountered the problem twice in 10 years.