Heat a grill to medium-high (about 450°). Put 1 tbsp. oil and the mustard seeds in a small frying pan. Cook, covered, over medium heat until you hear the first seed pop. Add cumin and let sizzle, covered, 1 minute. Remove from heat and let cool completely, about 10 minutes.
Meanwhile, brush salmon on both sides with remaining 1 tbsp. oil. Sprinkle flesh sides with 1/2 tsp. each salt and pepper. Grill salmon, flesh side down (close lid if using gas), carefully turning once with a wide spatula, until opaque but still moist-looking in thickest part (cut to test), 9 to 10 minutes. Lift salmon off skin onto serving plates.
In a food processor, pulse toasted seeds, 1 cup cilantro leaves and stems, the green onions, remaining 1 tsp. salt, the jalapeño, and garlic until finely chopped, 10 to 12 times. Add coconut milk and lime juice; pulse to combine.
Top each fillet with 1/4 cup sauce and 1 tbsp. cilantro leaves.
*Virgin coconut oil is minimally refined, leaving more nutrients (including antioxidants) intact than in regular refined coconut oil. Find it at well-stocked grocery stores.