Is the white fluid that comes out during sleep after dreaming of touching a little girl semen? Does its discharge in this case constitute masturbation and necessitate ghusl? And is there a solution to this problem?

As long as you see this in your sleep, the pen is lifted from the sleeper, so there is no accountability upon you for what you see. It appears that this fluid coming out of you is semen, which necessitates ghusl (definition) (ritual bathing). If you doubt the nature of this discharge, you have the option to assign it the ruling you wish. As for a way to prevent this from happening, we have no knowledge of it.