skin cancer vs melanoma

skin cancer and melanoma both are nouns.

Word	Noun	Adjective	Verb	Adverb
skin cancer	Yes	No	No	No
melanoma	Yes	No	No	No

As nouns, melanoma is a hyponym of skin cancer; that is, melanoma is a word with a more specific, narrower meaning than skin cancer:

skin cancer: a malignant neoplasm of the skin
melanoma: any of several malignant neoplasms (usually of the skin) consisting of melanocytes

Other hyponyms of skin cancer include cancroid, squamous cell carcinoma, epithelioma, malignant melanoma.

skin cancer (noun)	melanoma (noun)
a malignant neoplasm of the skin	any of several malignant neoplasms (usually of the skin) consisting of melanocytes