Halfway down the thread, I am 100% certain these are cattails. I have EXACTLY the same thing going on. In a new pond when the bottom soils have not accumulated any nutrients yet, the cattails are pretty pathetic and look more like grass. However if they manage to get some roots into something a little more fertile, they take off!
Pull them while they are little like this, and they will be easy to control. Wait too long and the tubers which travel just under the surface of the soils will snap off while you are pulling, and then you will have to pull again... and again...
