I'm no expert but it looks like the mold you get in damp conditions. It may not be deadly to the plants, but if it was me I'd remove the leaves as soon as I saw anything that resembled the sort of mold/dampness that I'd expect to see.

Mold usually has a "damp" smell about it, because in damp conditions we smell mold (and thus associate the smell of mold with dampness - i think ), does it smell like that?