Prepare wakame:
Combine wakame with warm water to cover by 1 inch and let stand 15 minutes, or until reconstituted.
Drain in a sieve.
Make soup:
Stir together miso and 1/2 cup dashi in a bowl until smooth.
Heat remaining dashi in a saucepan over moderately high heat until hot, then gently stir in tofu and reconstituted wakame.
Simmer 1 minute and remove from heat.
Immediately stir in miso mixture and scallion greens and serve