Cut lemongrass stalk in half crosswise; coarsely crush using a meat mallet or rolling pin.
Melt butter in a large saucepan over medium heat. Add potato, onion, celery, ginger, and garlic; cook 5 minutes or until celery and onion are tender, stirring occasionally. Stir in lemongrass, sugar, fish sauce, and clam juice. Bring to a boil; reduce heat, and simmer, uncovered, 10 minutes or until potato is tender.
Stir in zucchini, clams, and chiles. Bring to a boil; cook 2 minutes. Stir in coconut milk, lime juice, and cilantro; remove from heat. Remove lemongrass; discard.