You need plants and greenery to really complete any water feature, even a waterfall. But typical still water plants, like watercress or lilies, are not suitable for waterfalls unless you have a large pond that collects water at the base. Moss is one of the easiest things to grow on waterfalls because it thrives in a moist environment. The rocks surrounding the falls will be consistently damp, making it easy for moss spores to take and cling to your rocks. The best part about moss is that it can be vibrantly green and provide an almost instant "lived in" look to make your waterfall look much more natural. There are several ways to "plant" moss:

1. Bring a rock that already has moss established on the surface. Moss spreads with spores, so it does not have a real root system. Bringing a rock that already has a thriving moss culture will help the moss to spread to neighboring rocks. 
2. Bring moss to the rocks. If you're at a park or at a neighbors house and see some lovely moss growing, try to cut away a swath of moss from the rock and bring it back with you. Keep the sample moist and lay it on a wet area of the waterfall, out of the direct path of running water. With a little sunlight and consistent moisture, it should eventually behind to take hold of its new rock foundation and spread to other areas.
3. Paint your rocks with moss. Take some moss from another area of your yard (or even a nursery or nature trail). Blend the moss with some plain yogurt that has active cultures. The yogurt will provide food and the moss spores will be more easy to distribute over your rocks. Use a paintbrush to cover the rocks with the moss mixture. The moss should begin to grow back more uniformly over your waterfall area.

It's best to choose the right kinds of moss. Pillow moss, which grows in large pillowy clumps, is hardy and enjoys the spongy, wet environment of falling water. Fern moss is another popular moss type. Ferns look like plants, but they are actually mossy because they do not have roots. This moss will make it look like tiny ferns have started growing on your rocks for a more foresty feel that can blend in with neighboring shrubs. 

For more information about making your waterfall beautiful, contact a landscape service in your area like Colourscape Inc.