Invasive aquatic plants
It's tough to tell from the pictures whether this is a common beach grass or an invasive. Are there any areas where the plant is growing so you could send me a picture of the top? Then we can narrow in on what this is. Below is a web site with more info about invasive species the most common of which is Phragmites (Common Reed Grass). Note that this grows from 6-13 ft. tall so you should see stands that are easily recognizable.
There are many native aquatic grasses that will spread via underground rhizomes (underground roots) so a picture of the growing part would be helpful.
As for the the Japanese Knotweed you found in your yard, this is a challenging invasive to eradicate. As the article below indicates, with a multi-pronged, long-term approach, you can have success.
Hope that helps!