lichen vs skin disease

lichen and skin disease both are nouns.

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

As nouns, skin disease is a hypernym of lichen; that is, skin disease 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
skin disease: a disease affecting the skin

Other hypernyms of lichen include disease of the skin, skin disorder.

lichen (noun)	skin disease (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