The sticky substance on your hibiscus is likely honeydew, which is excreted by insects that suck on plant juices. Aphids, scale insects, and mealybugs all excrete honeydew. The means to get rid of the honeydew is to control the insects causing it. Aphids can be fairly easily controlled with insecticidal soap, or even a forceful spray of water. Mealybugs and scale insects are more difficult to control because they have a waxy coating that protects them. However, if you spray them at their crawler stage, you can get good control. It usually takes several spray cycles at week to 10 day intervals to get them under control.