Ruling 215. If the body of an animal becomes impure with an intrinsic impurity like blood, or with something that has become impure, such as impure water, in the event that the impurity is removed, the body of the animal becomes pure. Similarly, the inner parts of a human body – like the inside of the mouth, nose, and ears – become impure by coming into contact with an external impurity, but by removing the impurity they become pure. As for internal impurity – such as blood that comes out from in between the teeth – this does not cause the inner parts of the body to become impure. Similarly, if an external object inside the body comes into contact with internal impurity, it does not make the object impure. Therefore, if dentures come into contact with blood that comes out from in between the teeth, it is not necessary to wash the dentures; but if the dentures come into contact with impure food, it is necessary to wash them.

With regards to this rule. If I have something (external) najis in my mouth, but it is not in my mouth anymore, does my saliva after become najis?

Your Saliva is clean so long as the Najis is out and the mouth is cleaned with water.