Place the pistachios in the bowl of a food processor for five 1-second pulses to roughly chop them. Add the butter, parsley, cilantro, and panko. Process the mixture until the butter is well combined and the pistachios are finely ground, 30 to 45 seconds. Use a rubber spatula to scrape the mixture out onto the lower half of a long piece of plastic wrap. Fold the top portion of the plastic wrap over the butter, and gently roll and shape it into a 1 ½-inch-wide log. Twist the ends of the plastic to seal. Refrigerate for at least 2 hours or up to 5 days (or freeze for up to 3 months).
Pour the mango juice, orange juice, and passion fruit juice into a medium saucepan. Add the peppercorns and bring to a simmer over medium heat. Reduce the heat to medium-low and simmer gently until the liquid reduces by half, 20 to 25 minutes. Set aside to cool to room temperature.
Whisk the agave syrup and mustard into the juice mixture and then whisk in the vinegar. While whisking, slowly drizzle in the olive oil and then whisk in the salt. Pour the vinaigrette into an airtight container or glass jar and refrigerate. Shake well before using.
Prepare a hot charcoal or gas grill. (If using a charcoal grill, follow the manufacturer’s instructions to make a two-level fire, with one side of the grill at high heat and the other side at medium-low heat; if using a gas grill, adjust the burners so one side is at high heat and the other is at medium-low heat.)
To make the sea bass, set the fish on a rimmed baking sheet or on a large plate and drizzle with the olive oil. Sprinkle with the salt and pepper.
Slice the chilled pistachio butter into 4 pieces.
Grill the sea bass fillets flesh-side down until they have grill marks, about 5 minutes. Carefully turn over the fillets and reduce the heat to medium (if using a charcoal grill, flip over the fillets onto the cooler side). Place a piece of the pistachio butter on top of each fillet, cover the grill, and continue to cook the fillets until they flake easily and are opaque all the way through, about 5 minutes longer, depending on how thick they are. Remove them from the grill and set aside.
Divide the arugula among four plates. Place a piece of sea bass on top of the arugula, drizzle with the passion fruit vinaigrette, and serve.