Is the clear white fluid that comes out a week after menstruation, and then the white fluid accompanied by a little yellowness in the following week, considered semen (manī) or prostatic fluid (madhī)?

The cure for waswas (whispers/obsessive thoughts) is to ignore them. The difference between a woman's madhy (pre-ejaculatory fluid) and maniy (ejaculatory fluid) has been mentioned in previous fatwas. It appears that what the questioner described is neither madhy nor maniy, but rather normal, pure vaginal secretions, although they do invalidate ablution. As for yellowish discharge (sufrah), it is impure and requires purification (istinja') unless it is considered menstrual blood. To differentiate between yellowish discharge and vaginal secretions, you can refer to Fatwa no. 121284. If you are unsure about a discharge, you have the option to choose and assign it the ruling you prefer.