What is the fungus that grows on the mountain after rain?

The fungus that grows on the mountain after rain is lichen.

Lichen is a stable and mutually beneficial symbiotic relationship between fungi and photosynthetic organisms (green algae or cyanobacteria). Fungi are the main members, and their morphology and reproduction depend on fungi. In other words, lichen is a specialized fungus, which can be eaten. Lichen has a high nutritional value, containing a variety of amino acids and minerals, and the high content of calcium is rare in vegetables. As shown in the figure below:

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