Hi y'all!

I have another question. Some of my camellias have developed black stuff all over the leaves. It's not the leaves themselves that are blackening, it's a thin layer that you can scrape off with your fingernail. I'm not sure when it started. It doesn't seem to have affected the plants' health, cause they just finished blooming like crazy and they're putting out new growth. I almost want to say it looks like mildew, but I don't remember ever seeing this before. The leaves just look... dirty.

Is this something I should be worried about, or am I just being Sally Fusspot about a little dirt?