From what I've read, never actually kept them myself, it is an algae and will reproduce using spores. One way to trigger spores is to increase ammonia. But, we don't want to do that in a planted tank, do we? Otherwise, we might get more types of algae than just the marimo balls!
If you want more of just marimo balls, wait for it to grow enough so that the inside of the ball to become hollow, then propogation is done by splitting the ball and reforming smaller, solid balls. This takes a while, but it is better than triggering an ammonia spike!