It is one of those odd but peculiarly poetic and attractive words. Both 어슴 and 푸레 are obscure old base of it no longer in use, but "어슴푸레하다" is listed in dictionary. It means "dim"(아두운) or "faint"(희미한) but sounds way more poetic than those plain words. I wish I knew a word as nice as 어슴푸레하다 in English :-)

어슴푸레하다 means dim,vague, faint "소녀들은 어슴푸레한 달빛 속에서 조용히 춤을 추기 시작하였습니다." So, I think it means "The girls started to danced quietly in the dim moonlight."