Step 1In a medium bowl, cover seaweed with cold water. Let soak until rehydrated and softened, about 10 minutes. Drain and squeeze out any excess water. Cut with kitchen shears in bowl or chop into bite-size pieces on a work surface.
Step 2In a small bowl, whisk vinegar, mirin, oil, sesame seeds, soy sauce, and ginger. Drizzle dressing over seaweed and toss to coat, then fold in scallions.
Step 3Make Ahead: Seaweed salad can be made 3 days ahead. Transfer to an airtight container and chill.