It's most likely Hydrocotyle tripartita sp. Japan. That stuff grows real easy in moderate to high light. You can separate it into several pieces so long as there are at least 2 or 3 nodes on each stem and just plant. In higher light, it tends to grow lower to the substrate and creep along like a carpet. You can train it to grow in this fashion by pushing down the leaves that are trying to grow vertically. In lower light, it will just grow vertically and you can just trim accordingly. Replant the trimmings if you want to propagate it.