We are fighting a black film on our ficus tree's leaves. What is causing it and what do we do for it?

If that black film rubs off with a moist cloth or using your finger, then I suspect it is sooty mold growing on the exudate from scale insects or aphids. Scale insects will be on the uderside of the leaves and on the branches as well. Aphids typically are found on the underside of the leaves only. Another clue would be if leaves are sticky or shiny before the sooty mold develops. Any thing within and under the canopy could also be sticky or have the black coating. Identify the insect and then use an appropriate insecticide labeled for both the plant and the insect.

I think what you are seeing on the plant is sooty mold, a fungus that develops on "honeydew" that is secreted from insects such as aphids, scales, and mealybugs. These are all common pests of indoor plants. They do their damage by sucking juices from the plant and excrete a sticky material called honeydew. Aphids are fairly easy to see. They are soft-bodied insects. Scales can be visible as well, but can be a bit misleading because they can simply look like small brown bumps. Mealybug is quite obvious, looking like bits of cotton. In any case, they need to be controlled. Depending on the size of tree, taking it to a tub and showering the plant is a safe way to clean up the leaves. Insecticidal soaps are available at garden centers. Follow the directions. Other pesticides are available, but treatment would need to be done out-of-doors, and this is impossible at this time of year. Usually multiple treatments are needed since there the eggs are laid over several weeks. You need to control the insects in order to control the black mold.