They do not look like any hydra I have seen before though I have only seen a few of them. Those I have seen have a distinct white body that is very like a microworm, with the tentacles sprouting at the top, but when I can see the body clearly, I can also see the 'fingers' clearly. Needed a digital camera pic to see anything other than a worm like critter though. But the way they are standing up is a bit creepy for sure.
Might be fungal. Algae is usually green, can be brown, black or reddish, or blue green, but not white, unless it is stone dead. Fungus is often whitish.
Do they move at all ? Have you seen any of them close up the top part and move away, like a worm ? If not, try moving some of them. Swish them and see if you can get some to close up and move on. If so, they're likely hydra. You'd need a fish that eats them, gouramis are known to eat them, or drugs, I think it's a dog wormer that's used. I'd have to look up the one that kills them. Do not crush them, they can regenerate from even a single cell, if they are hydra.