In a two-quart pot, poach the oysters in their own liquid. Melt the butter in a large skillet on medium heat. Sauté the green onions, celery, garlic and parsley until translucent. Add the whipping cream. Peel the bloomy rind off the Brie, cut into small pieces, and add to the skillet. Raise the fire to high to enable the cheese to melt. Once the cheese is melted, lower the fire to a simmer and add the poached oysters with liquid. Add the half-and-half, salt, pepper, and cayenne and cook for 2 to 4 minutes. Add the potatoes, stir and turn off fire.