Rinse dried hijiki under cold running water, gently separating the strands with your fingers. Drain thoroughly.
Place rinsed hijiki in a large bowl and cover with 2 cups of water. Let soak for 20-30 minutes until fully rehydrated and tender. The seaweed will expand significantly.
While the hijiki soaks, prepare the dressing by whisking together soy sauce, rice vinegar, sesame oil, sugar, and grated ginger in a small bowl until sugar dissolves completely.
Drain the rehydrated hijiki through a fine-mesh strainer, then gently squeeze out excess water using clean hands.
Transfer the drained hijiki to a mixing bowl and pour about three-quarters of the prepared dressing over it. Toss gently to coat evenly and allow to marinate for 5-10 minutes.
Add the sliced cucumber and any optional ingredients (carrots, edamame, green onions) to the seasoned hijiki. Fold everything together gently.
Pour the remaining dressing over the salad and toss once more to ensure everything is well coated. Taste and adjust seasonings if necessary.
Transfer to a serving dish and sprinkle with toasted sesame seeds. For best flavor development, cover and refrigerate for at least 30 minutes before serving.
Just before serving, give the salad a final gentle toss and garnish with additional sesame seeds if desired.
