What is Dan? What does Ge Hong practice, Dan?

Waidan: It is made of medicine, which is called immortality, or prolonging life, eliminating all diseases and increasing vitality. The so-called fairy medicine elixir, and so on.

Inner Dan: It is the body of Dan formed by the true elements in the body (including all living things) through long-term self-cultivation. Like many enlightened people or animals that spit Dan, this Dan belongs to all the true elements of this creature. If lost, it will be life-threatening. There is also the inner Dan produced by the internal environment after death.

Either way, it is a high concentration of true elements.

Ge Hong can be an ancient chemist. Ancient times were to some extent the same as appellations, alchemists and chemists. It's just different times and cultures.