Hi Fraz, Welcome to ATP!
The first thing I thought of was slime mold too. Right now we have very high humidity and that stuff is showing up all over the place, but mostly on the cedar mulch. Full sun or part shade - doesn't seem to make any difference. I haven't found any near my sempervivum, but obviously they aren't mulched. I don't believe it does any harm, it just doesn't look very nice. After a few days it seems to disappear. You could probably just wash it off with a squirt from a hose.
It's common for mushrooms to grow near decaying stumps, so I wouldn't be concerned about that. Otherwise your sempervivum look very healthy.
