Will the righteous deeds performed by one who thought he had pre-seminal fluid (madhiy) and then discovered it was semen (maniy) be accepted from him, and is he required to repeat the prayer?

If it becomes clear that what was discharged from you necessitates ghusl, then you must perform ghusl and make up all the prayers you performed before purifying yourself from janabah. It is forbidden for someone in a state of janabah to recite the Qur'an or enter the mosque. However, there is no sin upon you for what you did, as you were mistaken, for the Prophet, peace and blessings be upon him, said: "Indeed, Allah has forgiven my Ummah for mistakes, forgetfulness, and what they are forced to do." As for the acceptance of deeds, that is a matter of the unseen.