What is this plant/creature in my freshwater aquarium?
That is a hydra.
They are a kind of jellyfish, and as such they do technically have stinging tentacles. However, they're also very small, so they're not a danger to most fish or shrimp. If you're planning on breeding your fish they will eat fish fry if they get a chance. On the other hand some fish species will eat them, so it's a bit of a wash.
Generally they only thrive in tanks with extra food hanging around, so you may consider feeding a little less than you have been.