1. Thaw fish, if frozen. Rinse fish; pat dry. Place fish, skin side down, in a shallow dish.
2. For rub, in a small bowl stir together sugar, lime peel, 1/2 teaspoon of the salt, and the red pepper. Sprinkle rub evenly over fish; rub in with your fingers. Cover and marinate in the refrigerator for 4 to 24 hours.
3. Meanwhile, for salsa, in a medium bowl combine mango, cucumber, green onions, lime juice, cilantro, jalapeno pepper, garlic, and the remaining 1/4 teaspoon salt. Cover and refrigerate until ready to serve.
4. In a grill with a cover, arrange medium-hot coals around a drip pan. Test for medium heat above the pan. Place fish, skin side down, on the greased grill rack over the drip pan, tucking under andy thin edges. Cover and grill for 20 to 25 minutes or until fish flakes easily with a fork. Serve with salsa.