It's duckweed, man!
Good luck trying to kill it!
Plant it in the substrate, and it'll grow. Cut it and plant the ends, it'll grow...and the bottom part will keep growing too. Heck, don't plant it and just let it float around the aquarium, and it'll grow!
My only advice is to try to keep roots whenever you can get them, because it'll be easier to keep the stalks down in the substrate.
Seriously, if you can find a way to kill duckweed, there's a few European cities that would LOVE to hear about it.