Answered by Ustadh Tariq Abdul-Rasheed
Question: If you wash the impurity off and the water (najas) runs down your leg is it necessary to wash that limb?
If washing a limb and the ad-mixture of water and filth runs down the limb then the area that is effected should be cleansed if one remembers and is able to remove it. This is based on the opinion that removing filth from body, clothing and place of prayer is an obligation if one remembers and is able to.
If one prays forgetfully or was able to remove the filth from their body, clothing or place of prayer (meaning the places where the limbs touch); but did not, it is recommended to re-make the prayer in its time. [Imam Dardir: Sharh Saghir]
Allah (Most High) knows best