Hanging may help- they are not photosynthetic, so it doesnt really matter what kind of lighting you have it under, as long as algae doesnt grow over top of it.
They require Extrordinary amounts of water flow, and constant feeding. Just what they eat though is a matter of debate. Search through the Advanced Reefkeeping forum for the Dendronephthya Study thread for good info. Try phytoplankton, stirring your sandbed regularly (daily + ), etc. Beautiful corals, good luck with it.