Micranthemum Micranthemoides
Pearlweed
This is a fairly easy to grow plant that just needs good light and substrate, with occasional fertiliser. We have it growing nicely in a tank without CO2
This is also a plant that changes quite a bit as it settles and develops underwater. It looks quite different when it arrives with us. 
Grows up to approx 20cm tall. Normally available in store and online in pots or invitro cups.