Four species of cordgrass, aka. Spartina, are highly invasive in Washington and many places worldwide. It alters local hydrology, negatively impacting habitats and recreation. While tough to ID, using apps such as iNaturalist can help greatly. 

Cordgrass features white, rice-shaped flowers along the top 6-8 inches of its stems. Leaves are narrow and pointy, but not tough. Pull seedlings & repeatedly cut down growth. 

(Image credit: King County Native Plant Society)