melanism vs disease of the skin

melanism and disease of the skin both are nouns.

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

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

melanism: a condition characterized by abnormal deposits of melanin (especially in the skin)
disease of the skin: a disease affecting the skin

Other hypernyms of melanism include skin disease, skin disorder.

melanism (noun)	disease of the skin (noun)
a condition characterized by abnormal deposits of melanin (especially in the skin)	a disease affecting the skin