1) Cover 2) Covering 3) Natural covering 
Crustose is a habit of some types of algae and lichens in which the plant grows tightly appressed to a substrate forming a biological layer of the adhering organism. Crustose adheres very closely to the substrates at all points. Crustose is found on rocks and tree bark.[5] Some species of marine algae of the Rhodophyta, in particular members of th...
[adj] - (of lichens) having a thin crusty thallus that adheres closely to the surface on which it is growing
adjective (of lichens) having a thin crusty thallus that adheres closely to the surface on which it is growing; `crustose lichens`
forming a crusty, tenaciously fixed mass that covers the surface on which it grows, as certain lichens. Cf. foliose, fruticose.