I saw this creature many times and while it kept itself on the leaf, it moved exactly how is described hydra moving - putting it's head down, then lifted it's foot and stepped onward, then again put head onward and moved foot and so on. And while hunting, it moved it's body here and there around, like dancing, not stepping off, lol.
Then, all of a sudden, it took itself off from the leaf and vibrate just as you described and was seen on the video, and I thaught - what the heck? Never heard nor read hydra moving like this!

Suddenly it acted like a worm or snake, and swimmed away. Couple of times more I saw it and was more and more confused. And I still am.

Hirudinea is flat indeed, that's why I thaught, it was a baby. When it vibrated, it seemed a bit flat too. Not like hydra anymore at all.