Hi Jonw37! I am not in SoCal, just a little nearer Norcal but with inland temps. Our temps here are soaring high and dry, so my semps also do that, so I have moved the containers in part sun/part shade. Depending on the orientation of your garden, if you can move them to a spot where in the afternoons it gets part shade, it will help. Or put a garden umbrella to shade them. I usually go around in the morning to spray/mist the plants while the sun has not hit them yet. Typically spraying the root zone to help cool them down, and even if I mist the leaves, it does not really matter, just too dry here anyways, so it quickly evaporates. Morning sun is okay, but as it goes hotter after 11am, it is just too toasty for the alpine succulents like sempervivums.
Most of my other succulents are doing that too, semps or not, closing their leaves when temps are hitting 90F and above. It is their means to conserve their water loss, so they go semi-dormant. I have only found two semps that really relish the heat, but still needs some shade, Sempervivum rubellum 'Mahogany' and Sempervivum kalinda.
As for the white stuff on the leaves, you can try a solution of alcohol and water, spray it on the leaves but do it when the sun is not yet hitting the plants. I have read somewhere some uses straight Listerine original formula.
