Serosa is entirely different from the adventitia, a connective tissue layer which binds together structures rather than reducing friction between them.
It is triggered when "collagen" from the subendothilium, and/or "tissue factor" from the media and adventitia bind with their respective receptors on the platelet.
The tunicae of blood vessels are 3 layers: an inner layer (the tunica intima), a middle layer (the tunica media), and an outer layer (the tunica adventitia).