Wet and Dry Semen on Clothing

Types of Impurity and Methods of Purification

وَالْمَنِيُّ نَجِسٌ، يَجِبُ غَسْلُ رَطْبِهِ، فَإِذَا جَفَّ عَلَى الثَّوْبِ أَجْزَأَ فِيهِ الْفَرْكُ.
Translation: Semen is considered impure; when wet it must be washed, but when it dries on a garment rubbing or scraping it off suffices.

Al-fark refers to rubbing or scraping away the dried substance. This applies when the dry material is actually removed; wet semen is washed.