crustose adj : (of lichens) having a thin crusty thallus that adheres closely to the surface on which it is growing; "crutose lichens"

crustose - having the form of a crust, as in the crust on a bread. Many marine algae grow closely appressed to a surface, and are referred to as crustose algae. Many lichens also grow close to the surface of rocks, tree trunks and other substrata, and are referred to as crustose lichens. Coralline algae are often crustose in habit.