Melt butter in a large sauté pan or skillet over medium. Add bell pepper, onion, celery, garlic, jalapeño, salt, and black pepper, and cook 6 minutes. Add paprika and cayenne pepper, and cook 1 more minute.
Place crawfish, cream cheese, Cheddar cheese, and cream in a 6-quart slow cooker. Stir in bell pepper mixture, Creole mustard, Worcestershire sauce, and hot sauce. Cover and cook on HIGH 45 minutes. Reduce heat to WARM. Stir in lemon juice and parsley; serve with crackers or toasted baguette slices.