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It was thought to be a Liverwort (like mosses in the Bryophyta), but DNA analysis has indicated that it is the gametophyte stage of a fern, and possibly a Lomariopsis sp., close to L. lineata.
So far no-one has induced it to grow its sporophyte stage (the bit we call a fern), so a definitive ID is still missing.
It's a great plant for breeding tanks, can grow in low lighting, and can even grow in brackish water up to 3 g/L (maybe higher)?
It does not cling to surfaces, so it's much easier to thin out than mosses. i use it in very small tanks with killies, Elassoma, Bettas, etc.
Monoselenium (= Pellia, a liverwort) has a midrib or vein through the center, often has rhizoids growing from the midrib, and a dichotomus branching pattern at the growing tips. It is relatively brittle and breaks when bent.
Lomariopsis (= Susswassertang, a fern) has no midrib, no rhizoids, more random growth structure (not regular dichotomus branching), and is softer and more flexible. I think this is what Gabriel has, based on no apparent midrib and rounded rather than dichotomus growth at tips. .
Need a close-up pic to be sure, but yes the overall appearance does look like Lomariopsis. In USA hobbyists called it "Round Pellia" before somebody figured out that it was really a fern gametophyte.