Roger Hamilton encounters an species alien to the mid-Atlantic that is numerous but not considered invasive. Indeed, it provides a valuable ecosystem function. It’s the filter-feeding freshwater clam Corbicula fluminea.
And, as the comment thread notes, this species (like almost any outsider) can indeed be harmful, depending on the habitat. It’s apparently rather a problem in Lake Tahoe, which is perhaps as different from the tidal Potomac as chalk from cheese.
ᔥ Leta