Does pre-seminal fluid (madhy) invalidate prayer if it exits during it, and what is the ruling on prayers that were performed while pre-seminal fluid was exiting, due to ignorance of that?

Everything that nullifies ablution also invalidates prayer if it occurs during it. You are required to wash the male organ and the garment from whatever has been affected by madhiy. If washing the garment is difficult, then sprinkling it with water is sufficient. As for the past prayers you performed while madhiy was exiting, due to ignorance of the ruling, the majority of scholars hold that they must be made up, based on the general statement of the Prophet (peace be upon him): "The prayer of one who has invalidated his ablution is not accepted until he performs ablution." Others hold that you are excused due to ignorance, by analogy with the hadith of the one who prayed incorrectly. However, adopting the first opinion is more appropriate and safer. If madhiy is exiting from you almost continuously, please refer to Fatwa No. 31631.