That is Utricularia gibba, its a bladderwort type plant. Those pods on the stems are traps which the plants uses to catch tiny micro critters. Quite an interesting feature in an aquatic plant.
This plant usually hitchhikes in with other plants. Its relatively harmless, just that it can grow fast and tangle into your other plants (like in your case). If you don't like it, just have to manually remove it strand by strand.
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