I was going to say Limnophila aromatica, until I saw that last picture. Mine doesn't have that appearance at any time - propagation is by side shoots, not little plants growing along a rooted stem or root. Now, I wonder if it is Pogostemon stellatus 'Broad Leaf'.
L. aromatica has a stem that is hollow, so if you cut a stem and check that, you should be able to tell.
I cut out the large dark plant in the photo. The stem was not hollow and had a very distinctive smell. The last photo of the 4" piece is floating at the surface putting off plantlets. But once planted, new shoots are sent out at the substrate level.