Combine pineapple, rice vinegar, sugar, Fresno chiles, 1/2 teaspoon of the salt, and 1 teaspoon of the oil in a small bowl. Cover and let stand 30 minutes.
Meanwhile, heat remaining 1 teaspoon oil in a large skillet over high. Pat tuna dry with a paper towel. Sprinkle tuna with pepper and remaining 1 teaspoon salt. Add tuna to skillet; cook to desired degree of doneness, about 2 minutes per side. Transfer tuna to a cutting board; cut across the grain into 1/4-inch-thick slices.
Divide seaweed salad and sliced tuna among tortillas. Top evenly with pineapple salsa, and sprinkle evenly with black sesame seeds.