These are jellyfish, the worlds smallest. They grow to a few mm. If you gently knock them off the glass using the water you will see them swim and it becomes obvious. They are really lovely and completely harmless.
They will start to pulse when suspended in just the water; I love them they are great. Use a syringe of water to see if you can blast them off, if they are the jellyfish they are not usually firmly attached to the glass.