What is the nature of the fluid that comes out after ghusl following masturbation and sticks to the underwear? Is it madhy, wady, or residual maniy? And it happens every time.

It appears that what is discharged is the remaining semen that exited due to masturbation. Scholars have differed regarding the obligation to repeat ghusl (ritual bath) if residual semen exits after ghusl without lust. There is no harm in acting upon the opinion of those who do not obligate repeating it, as it is a strong opinion. However, prudence dictates repeating ghusl to avoid scholarly disagreement. Semen is pure according to the preponderant opinion. Masturbation is Islamically forbidden except in cases of necessity. One must repel lust by fasting, avoiding solitude, associating with the righteous, attending lessons of knowledge, and striving for marriage.