Wasabi paste will heat up this simple bean salad, but it's not necessary.
Put beans in a medium saucepan and cover with cold water by 2 inches. Bring to a strong boil, then reduce heat and simmer, covered, until tender, about 1 hour. Add more boiling water if needed to maintain water level. When beans are tender, add salt to taste, and cook a few minutes longer. Drain and let cool.
Whisk together soy sauce, vinegar, wasabi, and pepper. Slowly whisk in oils. Combine with beans and refrigerate for at least 1 hour.
Rinse hijiki well in cold water. Soak for 20 minutes, drain, soak again for 1 minute, and rinse again. Set aside.
Arrange tatsoi on plates. Mix beans with cucumber and divide among plates. Garnish with hijiki and scallions.
Unlike most other dried beans, adzukis don't need to be soaked before cooking.