Preheat the oven to 425 degrees F (220 degrees C). Line a rimmed baking sheet with foil or parchment paper. Spray with cooking spray.
Pour melted butter over rockfish. Combine pecans, panko, and seafood seasoning in a bowl. Evenly distribute mixture over the fillets, pressing gently into the top.
Bake in the preheated oven until fish easily flakes with a fork, about 20 minutes. Set an oven rack about 6 inches from the heat source and turn on the oven's broiler. Broil for 1 to 2 minutes, making sure not to burn the crust. Serve immediately.