Must one repeat the Isha and Tarawih prayers because of forgetting to wash off white discharge (vaginal moisture) which the questioner thought was normal discharge? And what is the description of madhy (pre-seminal fluid)?

As long as the fluid is vaginal discharge, there is no need to wash it, because the preponderant opinion is that it is pure, even if it nullifies ablution. If you are uncertain whether the discharge is madhy (pre-seminal fluid) or vaginal discharge, you may choose to apply the ruling you prefer.