Is ghusl obligatory upon merely seeing the white discharge, or when seeing it completely free of blood, and is it necessary to make up the prayer if one performed ghusl and then the blood started flowing again?

The "white discharge" (al-qassah al-bayda') is a sign of purity, and it is a pure white fluid, like gypsum (white lime), that follows menstruation. Anything mixed with blood is not considered. A woman should not rush until she sees it. Once she sees it, she should perform ghusl (ritual bath) and pray. If it becomes clear to her that she missed prayers after this, she must make them up, and there is no sin upon her for not knowing.