Does it have little cups on it that look like they are filled with air? If so then you nailed it. A lot of emersed plants come with them as little nubs and they take off. I had one come in on a crypt pot and went from a 1/4" to 5" in a week. I ended up moving it out of the tank as it becomes a pain to later when it gets really stringy. Right now it's floating in a spare tank with some other hitchhiking floaters.