lichen vs disease of the skin

lichen and disease of the skin both are nouns.

Word	Noun	Adjective	Verb	Adverb
lichen	Yes	No	No	No
disease of the skin	Yes	No	No	No

As nouns, disease of the skin is a hypernym of lichen; that is, disease of the skin is a word with a broader meaning than lichen:

lichen: any of several eruptive skin diseases characterized by hard thick lesions grouped together and resembling lichens growing on rocks
disease of the skin: a disease affecting the skin

Other hypernyms of lichen include skin disease, skin disorder.

lichen (noun)	disease of the skin (noun)
any thallophytic plant of the division Lichenes; occur as crusty patches or bushy growths on tree trunks or rocks or bare ground etc.	a disease affecting the skin
any of several eruptive skin diseases characterized by hard thick lesions grouped together and resembling lichens growing on rocks