1. Preheat grill to medium-high heat.
2. Combine 1/2 teaspoon salt, black pepper, white pepper, and red pepper in a small bowl. Score steak with a sharp knife in a diamond pattern on both sides (1/4 inch deep). Lightly coat steak with cooking spray; sprinkle steak evenly with pepper mixture. Arrange steak on a grill rack coated with cooking spray; grill 4 minutes on each side or until desired degree of doneness. Let stand 5 minutes; cut steak across the grain into thin slices.
3. Arrange green onions on grill rack coated with cooking spray; grill 3 minutes or until slightly charred and tender.
4. Combine watermelon, mint, juice, oil, chile, and remaining 1/8 teaspoon salt in a medium bowl.
5. Place tortillas on grill rack coated with cooking spray; grill 1 minute on each side or until lightly charred. Arrange onions down the center of each tortilla; divide steak, salsa, and cheese evenly among tortillas.