Those are Hydra, not algae. They feed on microorganisms, which tend to abound in shrimp tanks because there are no fish eating them. Eradication is difficult in your situation because the meds to kill the Hydra will also kill the shrimp and snails.
They are a threat to shrimp and baby fish. They have a venom (or toxin) that immobilizes the prey and they feed from it. They also feed off baby brine shrimp which is why they are a rampant problem in fry tanks. I would at least take that one leaf away or kill them outside the tank in a bucket. I didn't use chemicals to eradicate the hydra in my nursery tank, but they killed my babies! I think I just dismantled the whole tank tbh.