The point at the tip of the 'leaf' reminds me of a moss/liverwort I got a couple of years ago. I'll see if I can find the ID I settled on.
Amblystegium serpens as I know it, tends to grow straight up and the 'leaves' alternate sides, so there are no 'leaves' directly opposite each other.
Worst case scenario, call it what you want, stick rare in front of the name and someone will buy it.
This one is another "leafy liverwort", again we aren't really in a position to say whether the plant sold as <"Solenostoma tetragonum"> is really that species, but S. tetragonum is also found on Borneo, growing with Bucephalandra spp.
Plagiomnium Affine would be my guess. In the pictures of P.affine I've seen, it also has those hair like growths seen in your photo (assuming that isn't algae). The growth habit doesn't look quite right though.
This is just a guess and as Darrel says, what we're sold something as doesn't always mean that's what it is and so there's a lot of incorrect IDs for mosses/liverworts out there.
I've grown quite some moss over the years and if growing it aquatic I always experienced mosses and filamentous alga seem to love each other they are about always together at some point. At least I thought so because it looked like it. Then I found this.
Moss spores germinate to form an alga-like filamentous structure called the protonema.