foster mother vs foster-mother

foster mother and foster-mother both are nouns.

Word	Noun	Adjective	Verb	Adverb
foster mother	Yes	No	No	No
foster-mother	Yes	No	No	No

As nouns, foster mother and foster-mother are synonyms defined as:

foster mother and foster-mother: a woman who is a foster parent and raises another's child

foster mother (noun)	foster-mother (noun)
a woman who is a foster parent and raises another's child	a woman who is a foster parent and raises another's child