pea soup vs pea-souper

pea soup and pea-souper both are nouns.

Word	Noun	Adjective	Verb	Adverb
pea soup	Yes	No	No	No
pea-souper	Yes	No	No	No

As nouns, pea soup and pea-souper are synonyms defined as:

pea soup (noun)	pea-souper (noun)
a thick soup made of dried peas (usually made into a puree)	a heavy thick yellow fog
a heavy thick yellow fog