Our hibiscus plant always has a grainy substance on the leaves. Hosing down doesn't help. I have to rub with water very heavily to remove this ugly stuff.
“Stuff” On Hibiscus Leaves
Certified GKH Gardening Expert
It could be scale, or some other type of insect pest. We recommend spraying neem oil to treat them. Here is information on scale and neem oil: https://www.gardeningknowhow.com/plant-problems/pests/scale-bug-how-to-control-plant-scale.htm, https://www.gardeningknowhow.com/problems/pests/neem-oil-uses.htm