Those are hydra. They can be an irritant to fish as well as the shrimp. The best thing I've found for them were spixi snails, they love the things (I've heard pond/bladder snails like em too, but I'm always reluctant to add those things to a tank). They're really interesting little critters, armed with tentacles lined with little stingers they use to kill and eat microorganisms. I would try cutting back on feeding a little bit and see if they disappear. After that, there's a couple other methods, including removing fish and heating up the water, or getting a fish that enjoys snacking on them. Although the last option might not be optimal, given your tank size.