They could also be Hemianthus micranthemoides (aka HM), very commonly mistaken for HC, especially when bought in emersed form in net pots from LFS. Sometimes they even hitchhike in with HC plants too, i've encountered it a few times.
Some info for reference (you can compare and see that the pot version in emersed form also have tiny round leaves): http://tropica.com/en/plants/plantdetails/4476/4476
HM are tall stem plants, so you can just shift and re-plant them in your tank's background instead.
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