Answer: I can't tell from your description whether you're dealing with an insect or a disease. Two things come to mind; if the white substance is like powder, it could be a fungal disease called powdery mildew; if the white substance is raised and bumpy, it could be mealybugs, or scale insects. In the case of powdery mildew, there are fungicides you can use to keep the disease from spreading, but it won't cure the leaves already infected. You may need to prune your plant back to remove the disease. This will also open it up to better air circulation. If you determine you're dealing with insects (try prying them off), you can use a Q-tip dipped in rubbing alcohol to kill them, or you can prune everything affected and dispose of the little critters.
Hope this information helps you determine the cause of the white substance on your hibiscus!
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