In a medium saucepan over medium heat, mix together milk, white wine, Cheddar cheese, Monterey Jack cheese and cream cheese. Cook, stirring frequently, until melted, about 10 minutes. Stir in green onions, spinach, dry mustard, cayenne pepper, garlic powder and black pepper. Continue cooking until all ingredients are well blended, about 10 minutes. Transfer mixture to a double boiler or fondue pot to keep warm while serving.