Melt butter in heavy large saucepan over medium heat. Add 3/4 cup green onions,- sauté 2 minutes. Add flour; whisk 1 minute. Gradually whisk in clam juice. Boil until mixture thickens, stirring often, about 3 minutes. Whisk in half and half and bring to boil. Boil I minute, stirring constantly. Reduce heat to low. Add both cheeses, horseradish, Worcestershire sauce and cayenne pepper-, stir until cheeses melt. (Can be made 1 day ahead. Cover and chill. Before continuing, rewarm over low heat, stirring constantly) Mix crab, remaining 1/2 cup parsley and remaining 1/4 cup green onions into cheese and stir over medium low heat to warm through.