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.countableuncountable

• 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.

Something which gradually spreads across something else, causing damage.countablefigurativelyuncountable

• 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.

To cover with lichen.transitive

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