In a medium saucepan, bring the water to a gentle simmer over medium heat.
While the water is heating, soak the dried wakame seaweed in a small bowl of warm water for about 10 minutes until it expands. Drain and set aside.
Once the water is simmering, reduce the heat to low. In a small bowl, mix the miso paste with a few tablespoons of the hot water until smooth.
Stir the miso mixture back into the saucepan with the simmering water.
Add the cubed tofu and soaked wakame to the pot. Allow everything to heat through for about 5 minutes, but do not let it boil.
If desired, stir in the soy sauce and sesame oil for added flavor.
Remove from heat and garnish with sliced green onions before serving.