I previously reported this European endangered* fern on other fora some 15 days ago when it had grown from 2 fronds to 5 fronds in a short time. The reason was partly that I was growing this portion in live Sphagnum which according to reports was not the medium to use. The plant is recorded as only colonising bare disturbed pond/lake edgings with no or very minimal competition from other plants. I think the photo speaks for itself, this plant is the healthiest of the ones I have.
Yes that is a fern , the US equivalent I believe is Pilularia americana which looks exactly the same.