lichen

Definitions:

One of cellular cryptogamous plants, which appear in the form of thin flat crusts, & c., covering rocks and the bark of trees; an obstinate and annoying popular affection of the skin.
- Nuttall's Standard dictionary of the English language. By Nuttall, P.Austin. Published 1914. (noun) (n.) (noun)
One of an order of air- nourished plants or fungi growing on stones, etc.
- The Winston Simplified Dictionary. By William Dodge Lewis, Edgar Arthur Singer. Published 1919. (noun) (n.) (noun)
One of the order of flowerless or cryptogamic plants found upon rocks and various bodies, commonly called rock or tree moss; a disease of the skin.
- Etymological and pronouncing dictionary of the English language. By Stormonth, James, Phelp, P. H. Published 1874. (noun) (n.) (noun)
One of an order of cellular flowerless plants: an eruption on the skin.
- The american dictionary of the english language. By Daniel Lyons. Published 1899. (noun) (n.) (noun)
A cellular flowerless plant.
- The Clarendon dictionary. By William Hand Browne, Samuel Stehman Haldeman. Published 1894. (noun) (n.) (noun)

Usage examples:

Halfway through the village the road passes through a gateway of solid stone, flanked by towers pierced for defense, and the wall through which this gate gives entrance remains, broken in places, lichen covered, yet still eloquent of its former strength and purpose.
- The Light That Lures by Percy Brebner
The old wooden, lichen covered rails had been taken away for a space of sixty or seventy feet; and, instead, a little distance farther back, on the Oak Dene land, a solid, brand- new fence had been erected; standing in a position which conveyed the impression that the sheltered nook to which- in her ignorance of boundaries- Miss Arnott had been so attached, and in which Mr Morice first discovered her, was part and parcel of Exham Park instead of Oak Dene.
- by
Did you ever see such green velvety lichen as seems growing on those old grey monoliths?
- Polly and Her Friends Abroad by Lillian Elizabeth Roy