The "Mercury Harp" is an item Sailor Mercury used in the manga and Crystal only. She used it to perform Mercury Aqua Rhapsody.
Manga[]
In the manga, she received it after her Sailor Power Guardian, Guardian Mercury, helped to release her from her nightmare.
Like the other items the other Inner Sailor Guardians received, the harp has a will of its own and can communicate with Sailor Mercury, but this was only shown once.