I think Charlie B may be right if the whiteflies or whitefly eggs/nymphs are there. If not though, another possibility is that they are experiencing sun bleaching. If they were recently transplanted from partial sun to full sun (especially on a rooftop) then they might be having a bad reaction to too much sun. To avoid this in the future, you can acclimate your plants to more and more sun over a few days. This often happens if you start your plants inside under lights and then move them outdoors.
Either way though, don't cut off the leaves that are showing the silvering. They are still functional and contributing something to photosynthesis, even if it is not as much as usual.