The process wherein blood forms clots are known as coagulation.
Blood coagulation or clotting is the mechanism to prevent excessive loss of blood from the body.
Reddish brown scum formed at the site of a cut is due to clot formed mainly of a network of threads called fibrins in which dead and damaged formed elements of blood are trapped.
The blood clotting proteins produce thrombin which is an enzyme that converts fibrinogen into fibrin and a reaction that results in forming the fibrin clot.