Does the dry white spot found on the underwear after dreaming of a handsome man constitute semen necessitating ghusl and making up prayers, knowing that the questioner is a girl and cannot distinguish semen?

Ghusl is obligatory if a woman is absolutely certain that semen has exited. If she doubts whether the discharge is semen or something else, she may choose and assign to it whatever ruling she wishes. Therefore, as long as she is not certain that the drop is semen, ghusl is not obligatory for her. Her performing wudu' for it is sufficient, and her prayers are valid.