Heat grapeseed oil over medium-high heat in a large pot. When it begins to shimmer, add sweet onion, ginger root and garlic and cook until the onion becomes translucent, being careful not to burn the garlic. Add the diced sweet potatoes and mango, stir to coat with the oil, and cook for about 5 minutes to allow the flavors to integrate. (Begin heating the grill for the marlin.) While stirring often with a wooden spoon, gradually add the coconut milk in increments, allowing the potatoes to absorb each addition of coconut milk before adding more. When the potato is cooked through but before it becomes mushy, remove from heat and let rest for 10 minutes. Microwave the lime in a small bowl to release the essential oils, and set aside until just cool enough to handle.
Brush oil on the marlin and season with salt and pepper and grill over medium-high heat, leaving each side undisturbed for the first few minutes to allow the seasonings to integrate into the surface of the fish and to allow the caramelization process to begin which will prevent tearing. Cook each side for about 4 to 6 minutes, remove from heat, and cover. If it is not cooked enough to your liking, you can finish in a preheated 375 degree F oven.
Squeeze lime juice over the risotto and stir in the scallions. (Note: If you keep the sliced side against your palm when you squeeze, the seeds are likely to stay in the rind.) Place some yam risotto in center of serving dish, and top with a piece of marlin. Garnish with a slice of lime.