I had a feeling it might have been Java Moss but I wasn't sure. I guess here are some more photos that are more detailed. Not sure if these help at all.
Front of the tank, near top of the tank by the light.
Another view of the front of the tank, near the top of the moss mass.
Near the top left of the tank, where the moss is starting to invade. They grow in strands and not really by branching much as far as I can tell.
Shot across the tank from the left to the right. The moss grows and arcs towards the light.
I guess for now, I'll assume it's Java Moss unless someone else thinks it's something else.