Hibiscus: HELP! Unknown hibiscus problem, 1 by JLD_II

JLD_II wrote:
Hello friends. Many of my tropical hibiscus have leaves that are rolling in to a cup like form. They also feel kind of brittle. I have what I think is a powdery mildew problem on some other plants, they look like they have hard water scale on the leaves. It wipes off with a lot of effort. Does that sound like Powdery Mildew and can it cause this kind of problem?
Here are a couple pictures of the hibiscus leaves. Please help me out with this.
Thanks, JD