Your hibiscus leaves have sooty mold on them. Sooty mold is caused by small insects that secrete a sugary substance on which the sooty mold grows. I suspect your insect problem is caused by white flies. These insects can be controlled by various insecticides. A good low toxicity insecticide is neem. You will need to spray it weekly for a period of time in order to achieve good control since the white flies life cycle must be broken. Your local Home Depot store can provide additional insecticide suggestions for controlling white flies.
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