Melt butter in a large saucepan over medium-high heat. Add onion, celery, and garlic; sauté 4 minutes. Add potato; sauté 1 minute. Sprinkle with flour; cook 1 minute, stirring constantly. Stir in milk, thyme, pepper, nutmeg, corn, and broth. Bring to a simmer over medium heat, stirring frequently. Cover, reduce heat, and simmer 20 minutes or until potato is tender, stirring occasionally. Stir in crab, parsley, and salt; cook 5 minutes, stirring occasionally.