Most likely you have one of the thousands of species of scale insect. Are the leaves kinda shinny and sticky to touch? A scale is a tiny insect which reproduces and feeds like an aphid. Unlike aphids, scale attach their feeding parts to leaf tissue, then build up layers of lac around their bodies as they grow. This means the insect is more protected than soft-bodied species, but you can still use any number of commercial sprays or systemic powders would work. I'm personally inclined to recommend Insecticidal soap (I use Safer brand), equal parts parts rubbing alcohol and water, or Neem oil diluted to the proper strength, since I try to avoid the really hardcore synthetics unless its a bleak prognosis. If you choose to use one of these products you have to respray a few times in the next few weeks to make sure all bugs were hit, and kill off anything which has hatched.
Peace,
Nate

Thanks for the information. So far, I've managed to control the mites by using a combination of water sprays and insecticidal soap. Recently, however, the plants seem to be under attack by caterpillars, including one white-ish leaf-rolling variety. Should I manually remove as many as possible, or simply sit back and hope that they will be controlled by natural predators eventually? I don't mind a few raggedy leaves, but I'm concerned that it might turn into an infestation and seriously threaten the hibiscus plants.