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It's a type of plant called a bladderwort. I believe yours is utricularia gibba. It's a pest not unlike duckweed. Do yourself a favor and remove all that you can.
Interesting fact, its actually a carnivorous plant.
Make sure you get all of it as one little piece left is enough to start it growing all over again. It can be a pain if it really gets established well and all over your tank. That happened to me many many years ago and I never thought I'd get rid of all of it. Have a little net handy to catch any little pieces that try to float away while removing it.