lichen
Any of many symbiotic organisms, being associations of algae and fungi, often found as white or yellow-to-blue–green patches on rocks, old walls, etc.
Other meanings
Something which gradually spreads across something else, causing damage.
Word history
From Ancient Greek "λειχήν".
Where this comes from
This definition is adapted from “lichen” on Wiktionary, by its contributors, published under CC BY-SA 4.0. This page is published under the same licence.