When nervous sometimes, there is a substance that comes down. Would it count as a pure or an impure substance?


Bismillahi Ta'ala,

Walaikum Assalam,
Indeed there are instances where women have reported a certain discharge due to nervousness or anxiety. What is difficult for me to assess is whether this discharge is from the cervix or outer vagina. As a primary measure, I request you to visit your gynaecologist to determine the source of this secretion as it could also be related to some other medical ailment. This answer is to help you until you are able to ascertain the source of your secretion.   
My dear colleague, Mufti Abdul Azeem bin Abdur Rahman Saheb sets the following criteria for assessment whether this discharge will require an ablution or not.
If a female is certain that the discharge she is experiencing is produced only in the vagina then such discharge is pure and does not nullify Wudhū. However this ruling is applicable on condition the discharge is clear or white (normal color). If it is mixed with blood or discharge due to arousal (Madhi) the Wudhu will be invalidated.
If a female is unaware of the exact origin of her discharge (the vagina or the cervix), then she must renew her Wudhū and purify herself before Salāh. This is so because of the possibility the discharge may be from the cervix and thus impure.
You may read his complete answer to ascertain the boundaries of cervix and vagina. (Click Here)
Wallahu A'lam,

Mufti Faisal al-Mahmudi