either water lettuce or parrots feather, common in today's plant trade for that floating plant.
Duckweed is great assets to some people as it does improve the water quality. However I am not crazy about em myself but I do try to keep few now and then and when they're getting out of control I just simply insert a cool whip tub and skim the surface and let the duckweed fall into the container when I bring the water level to the edges of the tub. I'm not sure if I'm explaining this clear at all. lol