That floating plant is not Cardamine lyrata, it is Lemna minor, commonly known as Duckweed. It's in our profiles. It is commonly called Duckweed because it occurs in ponds throughout North America and ducks love eating it. It is highly nutritious. Anyone with plant-eating fish (goldfish, Silver Dollar, Anostomus, etc) should have this plant.
I actually like this plant, as it has its uses. In new tanks or tanks with any water issues, it is superb. It grows very fast (as you've noticed
) which means it assimilates nutrients and toxins fast from the water. It is often recommended for new tanks or tanks with medications to help rid the tank water of the medication.
It often arrives on wood, other plants, even in the water with new fish. One near-invisible leaf alone will multiply. Each leaf whorl is an entire plant, and they reproduce fast. I have it in my 70g where I want it, and it has managed to spread to all the other tanks during water changes, moving a plant or fish, etc. I don't mind, as I can use a small fish net to remove it (or most of it) each week during the water change. If you want to completely rid the tank of it, you have to be sure of removing every tiniest spec of green floating on the surface. After a few weeks, it will be gone.