lichen

Etymology

Borrowed from Latin līchēn (“ringworm”), from Ancient Greek λειχήν (leikhḗn).

noun

1. 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.
   The nibble marks of the stone adze were still visible, though crusted over with scale lichens in most places. 1915, John Muir, chapter V, in Travels in Alaska
2. (figurative) Something which gradually spreads across something else, causing damage.