Firstly: if she has a regular cycle, and this discharge comes after the period has ended, then it is not part of her period, rather it is istihaadah (non-menstrual discharge), so she should not pay any attention to it, and she should do wudoo’ for every prayer and fast.
If she sees al-tuhr (purification), i.e., white discharge (when her period has ended), and then this discharge comes after that, she should not pay any attention to it, as in the first case.
If this happens during the days of her period, then it is part of her period, so she should not pray or fast.
Secondly: there is no specific du’aa’ to be said for a woman to be purified and end her period. See Question no. 12897.
Thirdly: the seat of the intention is the heart; it is not permitted to utter it aloud. So it is not correct for you to say before doing any righteous deed that you do, “I intend to do such and such…” because that was not narrated from the Prophet (peace and blessings of Allaah be upon him). The best of guidance is his guidance and every newly-invented matter is a bid’ah (innovation) and every innovation is a going astray. Please see Question no. 13337.