I'm assuming you know the time requirements involved for dwarfs seahorses, such as daily hatching of brine shrimp and two or three times a day feedings... so I won't go into details on how to set up a tank or care for dwarfs.
To answer your question, it is not advisable to use water, rock, decorations, macroalgae, etc from an existing marine aquarium as it could introduce hydroids and other non-wanted critters (such as amphipods) into the dwarf tank.
The best way to do it is to start with new water, new sand, new decor and fully cycle the tank for a few weeks before adding any dwarfs.
Tom