ice mass vs icecap

ice mass and icecap both are nouns.

Word	Noun	Adjective	Verb	Adverb
ice mass	Yes	No	No	No
icecap	Yes	No	No	No

As nouns, icecap is a hyponym of ice mass; that is, icecap is a word with a more specific, narrower meaning than ice mass:

ice mass: a large mass of ice
icecap: a mass of ice and snow that permanently covers a large area of land (e.g., the polar regions or a mountain peak)

Other hyponyms of ice mass include glacier, berg, iceberg, ice cap, ice field, floe, ice floe.

ice mass (noun)	icecap (noun)
a large mass of ice	a mass of ice and snow that permanently covers a large area of land (e.g., the polar regions or a mountain peak)