Is the adjective snow-capped or snow capped?

Answer 1

The adjective should properly be hyphenated, as snow-capped, because the noun adjunct form is not different or unique. However, some dictionaries do list it as a single word "snowcapped" from the equally rare noun "snowcap."

Answer 2

Yes, the hyphenated form snow-covered is an adjective, or an adjectival form at least. In the sentence 'the snow covered the sidewalk' it would not be an adjective.