white splotches on plant

two of my plants have a white substance on a leaf each. It's not overpowering them and running rampant but it is worrying me because it has happened to more than plant. It almost looks like something dried up on it. It probably came from a nutrient deficieny, could this be the case?

I suspect that it could be powdery mildew....

is powdery mildew something I need to worry about or try to clean off?

It will not be fatal overnight but you are gonna want to control it before it can get well established.

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Good luck