Is it necessary to repeat the ablution, or is it sufficient to wash the area of the madhy for someone who discovers traces of madhy after performing ablution and prayer?

You do not need to repeat your ablution if you find pre-seminal fluid (madhy) on your thigh after performing ablution and prayer. Rather, you only need to purify the spot. Your prayer is valid according to the majority of scholars, as long as you were unaware of the impurity before the prayer.

If you had not cleansed yourself from the pre-seminal fluid (istinja') before ablution, then it is obligatory to perform istinja', but your ablution is valid because istinja' is not a condition for the validity of ablution.

As for someone who prayed with the impurity of pre-seminal fluid without being aware of it, their prayer is valid according to many scholars, although it is more cautious to repeat it.