Cacti can be less hardy than you might think. If you can kill mint, then worry. *g*

Senecio radicans is pretty tolerant of aridity -- you're better off letting it dry out slightly (slightly *g*) between waterings than making it too soggy. Once a week is probably about right, but maybe use a little less water each time? The mold won't hurt it, I don't think, but letting the pot dry out a little should discourage it.