Not trying to be a smart ass here, but why are you certain this could not be hard water deposits? That was actually my very first thought looking at the photo, as I also have hard water, and sometimes see this with calcium on strawberry leaves, and on dried dishes after hand-washing them and leaving them to air dry. If it’s mildew, spider mites, or something similar, you can try methods such as spray on neem oil, or a diluted hydrogen peroxide solution to eradicate the disease without harming to plant. Best of luck whatever the case may be for you.