I have gorgonias and zoas in my tank along with different types of
macro algae. Mushrooms and leathers,
kenya trees these types would also be fine. Stay away from any stinging corals. Clams can and will close up and kill a seahorse or a fish for that matter so I would also avoid them.
You just have to remember that when a seahorse gets scared it tries to become invisible, So if it hitches to something that hurts it, instead of fleeing it will tighten its grip with its tail and try to disappear.
Hope that helps.
Jan