I'm sorry, Vinh...I don't know what it is either. I have a lot of it growing around my pond and I pull it whenever I see it. Not because I'm afraid they'll eat it, but because I don't like the looks of it.
I've got it growing around here, too, but never had a good ID for it...found it w/help of this database (http://weedid.wisc.edu/ca/weedid.php which someone was awesome enough to post on this forum somewhere a few weeks back :