Yes Clam Chowder is considered to be a soup but some people consider Clam Chowder to be a stew.
Either way Clam Chowder can be considered to be a soup or stew because really it is considered both.
Chowders are actually just thicker soups so if you have any type of chowder you really have a thick soup or thick stew.
Chowder is a type of soup or stew often prepared with milk or cream and thickened with broken crackers, crushed ship biscuit, or a roux.
New England clam chowder is typically made with chopped clams and diced potatoes, in a mixed cream and milk base, often with a small amount of butter.