Assalaamu alaykum! Say that after having a wet dream, I find that there are also some spots of Mathi (pre-ejaculate), and I am sure about it being Mathi on dried semen in the cloth - I know that semen is pure and that sprinkling water over cloth having Mathi makes it pure - but does this also apply to my case, in which there is some dried semen and a spot of Mathi together? Or must I wash in this case?

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger. 

As long as you are sure of the presence of Mathi, then you must purify the garment in which you want to perform the prayer from the Mathi, whether the Mathi is by itself or it is with semen, and whether the Mathi is dry or wet, because the reason for purification is the presence of an impurity, and Mathi is impure. So you must purify the garment from it, regardless of whether or not it is with semen.

In purifying it, it is better to wash it and not only sprinkle water over it, because some scholars are of the view that sprinkling water is not sufficient.

An-Nawawi said:

“The nation unanimously agreed that Mathi and Wadi (a thick, white secretion which occurs immediately after uniration by some people) are impure. Our view and the view of the majority of the scholars is that it is a must to wash the Mathi, and it is not enough to just splash water over it. Ahmad ibn Hanbal, may Allah have mercy on him, said, ‘I hope that splashing water over it is sufficient.’ He used the narration in Saheeh Muslim, which reads, ‘Perform ablution, and splash water over your private parts,’ as evidence. Our evidence is the narration which reads, 'Wash your private parts and perform ablution', and it is the one that is narrated the most, and by analogy (this extends) to all other impurities. As for the narration of splashing water, it is understood to mean washing.”

Allah knows best.