Rinse clams, and place them in a heavy stockpot. Add 5 cups of water, ginger, and dashi-no-moto; bring to a rapid boil.
In the meantime, place miso paste in a bowl, and dilute with 1 cup water to avoid lumps in the soup. When soup comes to a boil, add the miso and wakame. Salt to taste. Simmer for 2 minutes, and serve hot. Garnish with chopped green onions.