Question

Why does bleeding stop by rubbing moist alum?

Answer 1

Blood is a colloidal solution of an albuminoid substance. So, it contains charged protein molecules.

Moist alum has highly charged $A{l}^{+3}\text{and}S{O}_4^{2-}$ ions which neutralize the charged protein molecules present in blood. This results in coagulation of blood, forming a clot which stops further bleeding.