Usually in plants u buy there are worms snails and things like that as i know what u can do is to remove the whole water wash the plant wit some fresh water and then add it to the tank that way eggs and thing like that wont get into your tank
I think it's safe to assume there are others hiding. I went to a lake this weekend and collected a handful of duckweed. There were over 100 planaria in there. This to tell you that even if you don't see them there might be lots of them.
I would wash in tap water (hopefully they die with chlorine? Not sure) and leave it in a quarantine bucket for a while and check for any signs of life. I know fish eat tubifex but if it's planaria, they can harm fry and shrimplets