Time. Medium light with short photoperiod to prevent algae. Once you get algae on your moss you're done. 6 hours max. It takes a while tho, the moss grows faster in the beginning when the soil is still leaching ammonia.
Question, does it mean I shouldn't grow weeping moss with some hair algae on it? I'll put transferring it into my new tank, just not sure if I should do it. It has very quick grow though. (moss)