Preheat the oven broiler. Cover a medium baking dish with foil.
Melt butter with cream in a saucepan over medium heat, and blend in Creole mustard. Cook and stir until thickened. Mix in crab and shrimp. Season with Cajun seasoning, salt, and pepper. Cook until heated through.
Place grouper in the prepared baking dish, and rub with olive oil, lemon juice, and parsley. Season with salt and pepper.
Broil grouper about 4 minutes on each side, until easily flaked with a fork. Spoon the crab and shrimp mixture over fish to serve.