If I remember correctly moss is a stage in colonisation. Algae is sometimes first and provides a base for lichens to grow in. The lichens break down the rock and create and add to it as they decompose to make a layer of soil.
Then eventually when there is enough soil and nutrients moss moves in. They slowly take over the lichens and do well on the minimal amount of water there.
As the moss grows more soil is created and it also starts to hold more water until eventually you have an environment that more complex plants like grasses and herbs can take root in which may or may not overtake the moss eventually.
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