Are you referring to a free-floating moss or a species-specific carpeting plant? The latter of the two will require good lighting, a nutrient rich substrate, proper filtration, regular fertilizing, co2 injection (often required w/ high lighting). There are a lot of good carpeting plant options (S. Repens, Dwarf Sag, Dwarf Hairgrass, etc.). As for moss, you could throw some Java Moss in there, tie it to the lower driftwood and wait (and wait, and wait, and wait) for it to eventually take over the lower half of the tank. Again though, this will require decent lighting and regular ferts to happen. Otherwise, the Java Moss will either die off or take years to fill in enough to cover the bottom.
Also, I'd recommend a substrate in your tank.