Augh! I just remembered!
I made a topic over a year ago about small carnivorous plants that I had incidentally introduced into my tank! I cannot for the life of me remember what they were called, but whatever these things are, that's what the plants were. They grew from long thin vines and fed on microscopic organisms and very small fry...
That's gotta be what these are, because they look exactly the same.
....Or maybe I have both. Ugh!
Edit: Let me make it clear as to why I'm skeptical to believe this is hydra:
Every picture I've seen marks hydra as splitting from the stalk. These guys sprout as one head on one stalk and have showed no signs of ever separating from the stalk.
The tentacle things are much shorter than any picture of seen of hydra.
They don't have a crown of tentacles on top of the head, they have a smattering of tentacles all over the head.
From what I've seen, hydra grow independent of each other. This stuff grows out of a long vine. I don't know what the stuff in the ten is, but the growth in the five looks exactly the same as the stuff in the 40, and it all originates from a vine.
Sooo....there?