Try Java moss. It is readily available from stores that sell tropical fish and related supplies and plants.

There is a dwarf form of Java moss that you might have to hunt for..very attractive plant,an improvement over the regular form.

Thanks for suggestions. I guess java moss never occurred to me since the plant would not be actually submerged, but nearly so. Certainly worth a shot. What would the best way to secure the moss to the concrete be? It's not really something I can tie a string around.

Thanks,
Bryan

Probably super glue gel would work. If not, Moss tied to a stick of driftwood leaned or placed flat to the background would work.But the concrete has to be CONSTANLY wet or the moss wont attach.