What is the fluid that comes out after bathing from a wet dream? Is it seminal residue? Is it pure or impure? What should be done if a person touches its spot after it dries?

First, masturbation is forbidden. If you are unsure whether the fluid discharged during sleep is manī (seminal fluid) or madhī (pre-seminal fluid), you may consider it either. If you consider it manī, it is pure according to the majority of scholars, and there is no harm in touching its location after it has dried or sleeping on the bedding it touched. However, if you are certain it is madhī, then ghusl (ritual bath) is not required; rather, wudu' (ablution) is required, along with washing the penis and the affected area of the body, clothing, and bedding.

If a fluid is discharged after performing ghusl from janābah (state of ritual impurity requiring ghusl), if it has the characteristics of manī, then it is manī; and if it has the characteristics of madhī, then it is madhī. If you are unsure, you may consider it either. The view that ghusl is required a second time for manī discharged after the initial ghusl is the more cautious approach, and it is the opinion of the Shafi'i school and a group of scholars, while others hold that ghusl is not obligatory.