The white clover is Nancy's motif throughout the game. The official tracklisting on the album calls it "しろつめく" which I believe should be translated as "white clover", not "white pearlwort". The white clover appears in the form of the necklace, as well as the field that Alyssa wakes up in at the end of the game. It only makes since that it would also be the name of the song that Nancy sings to her, and not some other flower.