Last year, found among other plants from Southeast Asia,
light green color and has white roots. The photo shows part of the plant (emerse) and about 4in. long stems, growing under water (submerse).
IMO it's a Selaginella of some kind. Interesting to see how thin submersed stems are grown from the emersed ones.
Species ID might be very difficult, but SE Asia may be a valuable information. There are many Selaginella species worldwide, mainly tropical/subtropical. Surely only emersed plants can be IDed because the submersed ones look untypical.
Is the growth of the submersed stems erect or rather creeping? I wonder if the submersed stems always grow that thin, or if they get denser and broader, i.e. more similar to the terrestrial stems, with increased light/nutrient/CO2 supply.