What should be done after performing ghusl for madhy (pre-seminal fluid) and then it becomes clear that it was maniy (seminal fluid) after sunrise?

If you ascertain that semen has been discharged and you prayed without performing ghusl, then you must perform ghusl al-janaba (ritual bath after major impurity) and repeat the Fajr prayer, because it is invalid due to the absence of the condition of major ritual purity. You must also repeat all prayers you performed before performing ghusl from janaba. This is the view of the majority of scholars and is the more preponderant opinion. However, Sheikh al-Islam Ibn Taymiyyah held that whoever neglects a condition or a pillar due to forgetfulness or ignorance is not obliged to repeat the prayer. According to this view, your prayer would be valid. Nevertheless, the opinion of the majority is more cautious and more absolving of responsibility.