It looks like it is either whitefly, mealybug, aphid or spider mites. Even though you cannot see them they are there. The black spots are sooty mold that is forming on the honeydew that is being excreted by the insect. There are many different chemicals that you can use, or you can try washing them off by spraying the plant with a stream of water. It may take a couple of times of spraying the plant down to remove them completely.

Thanks,