Hibiscus are prone to pests. Put it in the shower/tub and spray it really well with Safer's Soap. All of the leaves, both sides. Do this every week for a few weeks. Putting it in a warm shower every week is a good way to keep them healthy too...just make sure that it can drain really well and does not sit in water!

From my personal experience with pests on both my hibiscus outside and the tropical ones I keep inside, I think it might be whiteflies.They are little tiny white flying insects(easy to miss) and their larvae are greenish and, again tiny, under the leaves usually that feed on the plant and secrete waste that cause the leaves to turn yellow or mold(might be the reason behind the spots). I can't say that's it for sure but it the only pest that has really showed up on my hibiscus. I would look for insecticide soap and I second alibraun's suggestions. I usually do the same thing with mine.
Good luck!
~mike

You are in Zone 10 as I am, is there some reason why you do not plant your hibiscus outside? I don't see them doing well in pot's for very long, and to give them a shower, I would not advise, as hibiscus are tropical and DO NOT like wet feet, they actually prefer to be dry with a light watering maybe once a week...
My pink:{{gwi:883740}}