Mix light brown sugar, white pepper, cilantro, soy sauce, oyster sauce, and garlic together in a bowl. Add salmon and marinate for 15 minutes.
Preheat an outdoor grill for medium heat and lightly oil the grate.
Beginning and ending with salmon, thread salmon, green pepper, red pepper, and red onions onto 4 skewers.
Cook salmon skewers on the hot grill until fish is opaque in color, turning every 5 minutes, about 15 minutes total.
To each tortilla add coleslaw, cucumbers, and radishes and top with salmon. Garnish with sliced green onions.