melanism vs skin disease

melanism and skin disease both are nouns.

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

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

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

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

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