first of all, before you talk which kind of corals are safe, you need to talk light. Very intense metal halide lighting is not a seahorses favorite thing. Maybe a 20k would make it a little more pleasent, but they really don't like real bright lights.
I say get some high wattage compact florescents and grow some beautifuly colored macro with your seahorses.
DOn't Seahorse safe corals, include the corals that have stinging tenticles, or tenticles that come out at night.
Things such as the colt coral/tree coral.. mushrooms, polyps, gorgonians, pulsing xenia, toadstool leather, sponges.
Clams are usually not good with seahorses, they can drag their tailing across the clam and it will close up, trapping the tail.
Hope this helps.