There is a black, tiny insect infestation under and around new leaves and buds of hibiscus indoors. My mature hibiscus plant recently developed tiny dark spots, lots of them, under leaves and new growth. Used insecticidal soap and cut plant back to only new buds but it's reappearing on new leaves.
Tiny Black Bugs on Hibiscus
Certified GKH Gardening Expert
It may be aphids. This article will help:
It might also be several other bugs, but aphids are the most common. Regardless of the bug, all of the common pests on hibiscus can be treated with neem oil. Here is more information on neem oil: