Place noodles in a large bowl and pour boiling water over to cover. Let noodles soak until tender but not mushy, 10–15 minutes; drain. Rinse under cold water; snip into 2" lengths with kitchen shears and return to bowl. Meanwhile, soak arame in 1/2 cup cold water in a small bowl until softened, 8–10 minutes. Drain; set aside. Whisk vinegar, soy sauce, mirin, lemon juice, chile, and oil in a large bowl. Transfer ⅓ cup lemon-chile shoyu to bowl with noodles and toss to coat. Add pear, sweet onion, yellowtail, mint, chives, reserved arame, and 2 tsp. sesame seeds to remaining shoyu in large bowl and gently toss; season with salt. Divide noodles among bowls and top with yellowtail mixture; sprinkle with gochugaru and more sesame seeds. Shoyu can be made 1 day ahead. Cover and chill.