Place wakame and hijiki in separate small bowls. Cover with boiling water and let rest until rehydrated and tender, about 5 minutes. Drain and press with paper towels. Roughly chop wakame. Add wakame and hijiki to a large bowl.
Add tuna, onion, scallion, sesame seeds, soy sauce, sesame oil, honey, and optional crushed red pepper to bowl. Season with a small pinch of kosher salt and gently fold to combine. Taste and adjust with more soy sauce, sesame oil, honey, or crushed red pepper as desired. Let sit 5 minutes at room temperature, then serve on its own or on top of steamed rice.
