This moss was originally bought under the the name Java Moss. However, the pattern of growth and the structure of its leaves seem to suggest that it is not the common Java moss (Taxiphyllum barbieri) but rather the true Vesicularia species of either Christmass moss (V. montagne, info) or Weeping moss (V. ferriei, info). From the look of the plant in my tank, it looks more like Weeping moss but the closeup of leaf and cell structures (see microscope photos here) seem to suggest that it is Christmass moss, according to an article (Warning!! 6.8MB!!) on various types of aquatic mosses written by Prof Tan from Singapore. Which ever the true ID of this moss, I have managed to grown them attached to driftwood and they tend to grow downwards as shown in the photo below.