The "Horn Coral" is also known as the Hydnophora. Most likely this coral you're talking about is Hydophora Rigida. From my experience, it's been quite a hardy coral, and a fast grower at that. I got a small frag of Hydnophora back in August, the size of a pencil eraser. Since then, it's grown about 2-3"
I personally wouldn't keep it under anything but Metal Halides. Mine hasn't showed much growth at all under Power Compacts or VHO's, and the coloration was very weak. They do best under metal halides, and in a mature tank. In my opinion, and only in my opinion, this coral should be added to an aquarium which is 7 months or older.
FWIW,
This is a small polyp scleractinian, which will need a stable calcium level, preferably of around 400-500ppm, as the coral is calcium carbonate based.
Good Luck,
Graham