melanism vs skin disorder

melanism and skin disorder both are nouns.

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

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

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

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

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