lichen

noun

1. (countable, uncountable)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 Beaches of Lukannon–the winter wheat so tall, / The dripping, crinkled lichens, and the sea-fog drenching all!”

   “It was the same rich green that one sees on forest moss or on the lichen in caves: plants which like these grow in a perpetual twilight.”

   “The nibble marks of the stone adze were still visible, though crusted over with scale lichens in most places.”

2. (countable, figuratively, uncountable)Something which gradually spreads across something else, causing damage.

   “Meanwhile, abiding a day of judgment, she fought ceaselessly to deny the bitter drops in her cup, to tear back the slow, the intangibly slow growth of a hot, corrosive lichen eating into her heart.”

verb

1. (transitive)To cover with lichen.

   “[…] making the rocks assume the mould of age and lichening the trees with damp beauty.”