In layman/hobbiest terms : yes it is what is called a "sperm plug".

Technically it is not. As stated above a sperm plug is a waxy deposit that a male leaves after mating with a female to prevent another males sperm from reaching the females oviducts. Garter Snakes use sperm plugs.

What we see in Ball Pythons should be called a hemipenial "cast". It is dried up hemipenial shed,dried mucus with probably a dash of the reptilian equivilant to smegma.