If I noticed a Najas (impurity) on my dress after finishing my prayer (Salat) will my prayer be valid or do I have to repeat it after cleaning the Najas?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

If you do not know whether the impurity occurred during the prayer or after it, then your prayer is valid and you do not have to repeat it. Imam al-Nawawi said: 'if the person ends his prayer with the Salaam then notices some impurity that could have been there during the prayer or could have occurred after the prayer, then his prayer is valid. There is no disagreement on this opinion among scholars of al-Shafi'e Madhab. al-Shafi'e and Shafi'e scholars say: 'it is likable for him to pray that prayer once again, just to remain on the safe side'.
But, if he notices that it was there during the prayer there are two opinions. The most preponderant one is that his prayer is invalid. The second opinion is that his prayer is valid. The Hanbalites said the same thing as the Shafi'etes.
But, if the person notices some impurity while praying, he can remove it without much movement, otherwise his prayer is invalid and he has to repeat it.
Allah knows best.