Fissidens nobilis
One of the first fissidens I've ever had, this is a tiny plant that almost did not survive when I replanted my tank.
It is relatively easy to keep, and although a lot of it melted or turned brown when I put them in the new tank, this new growth as seen in the photo took only 3 months to achieve. This "ball" of the plant is almost 2" thick! It is currently doing well under semi-direct light.