What is the yellowish substance resembling semen that is discharged after a wet dream, and what are its causes and harms?

If a Muslim experiences a wet dream and finds moisture, then if it is confirmed to be mani (seminal fluid), ghusl (ritual bath) is obligatory. If it is confirmed to be madhiy (pre-seminal fluid), wudu (ablution) is obligatory, along with purifying the body and clothes. In case of doubt between mani and madhiy, it is detailed as explained in Fatwa No. 98077. If the fluid found possesses the characteristics of mani, then it is apparent that it is mani and necessitates ghusl, and the absence of some of its characteristics does not matter, as they might be absent due to a temporary reason. As Al-Nawawi said: "The mani of a man in a state of health is white, thick, gushes out in successive دفعات (spurts), exits with desire, and its exit is accompanied by pleasure. Then, after its exit, it is followed by languor. Its smell is like the smell of palm spathe, close to the smell of dough, and when it dries, its smell is like the smell of eggs. These are its characteristics, but some of them may be absent, yet it remains mani necessitating ghusl, such as it becoming thin and yellowish due to illness, or exiting without desire or pleasure due to the relaxation of its receptacle, or turning reddish due to frequent intercourse and becoming like meat water, or sometimes exiting as fresh blood. And it is pure, necessitating ghusl." As for its cause, whether it has harms, or how to treat it, you can consult the counseling section.