Lichen

Today's the day ………………………… to explain

A lichen is a composite organism that emerges from algae or cyanobacteria living among the filaments (hyphae) of fungi in a mutually beneficial symbiotic relationship. The fungi benefit from the carbohydrates produced by the algae or cyanobacteria via photosynthesis.

And in the unlikely event that you need more of an explanation, you'll find more here ………………………….!