Melt butter in large skillet over medium-high heat. Sprinkle scallops with salt and pepper. Add to skillet; sauté until just opaque in center, about 5 minutes. Using slotted spoon, transfer scallops to bowl.
Reduce heat to medium. Add mushrooms, 1/2 cup green onions and cayenne to skillet. Sauté until mushrooms are tender and brown, about 6 minutes. Sprinkle flour into skillet; stir 30 seconds. Add clam juice and cream; simmer until sauce thickens enough to coat spoon, stirring occasionally, about 10 minutes. Add scallops. Stir until heated through, about 1 minute. Season with salt and pepper.
Place rice in shallow bowl; top with scallop mixture, then 1/4 cup green onions.
