Scale Insects On Hibiscus

This looks to be either Cottony Cushion Scale, or Newman Snow Scale, or Lesser Snow Scale. All of these scale insects are plant sap-suckers and can be very damaging to Hibiscus if their populations are high (numerous insects causing leaves to yellow and fall off, or many scales on the stems). If these are the only ones you've found, just seal the leaves up and throw them in the trash and continue to keep an eye out for more of them by inspecting the plant (other plants close by and other similar plants) on a regular (weekly) basis. If the population is large, we recommend you seek advice from local Master Gardeners (there may be a group associated with your regional agriculature/horticulture University or County Ag Dept. You could also check with your local nursery. One more note - it looks like there may be one or two scale (on the leaves to the right of the photo) that are swollen and dark in color? If so, these may have been parasitized by another insect (referred to in general as natural enemies) that use these scale as a host to lay its eggs in. That's a very good thing to help you maintain control of the scale, so don't use pesticides because those products kill the natural enemies too.