Is it obligatory for someone who masturbated during Ramadan, thinking that what came out was madhy (pre-seminal fluid) which does not necessitate a ritual bath (ghusl), but then it became clear to him that it was maniyy (seminal fluid), to make up for the prayers and fasting?

The first duties of a mukallaf (accountable person) after believing are to know the rulings of purity (tahara), prayer (salat), and fasting (sawm). Scholars have differed regarding the make-up (qada') of fasting and prayer for someone excused by ignorance. The more preponderant opinion is that make-up is not obligatory, based on the story of 'Adī ibn Ḥātim with the Prophet, may Allah bless him and grant him peace. However, whoever makes them up as a precaution and to avoid disagreement, that is more cautious.