A science fiction book with this title was published a number of years before The Alchemist.  Stranger in a Strange Land by Robert A. Heinlein was published in 1961.  It tells the story of a man raised by Martians on Mars returning to Earth, where he transforms the culture.

The phrase comes from Exodus:

Moses was content to stay with the man, who gave his daughter Zipporah to Moses in marriage. And she bore him a son, and he called him Gershom, saying, "I have been a stranger in a strange land".

Exodus 2:21-22

Gershom means "exile".

          

 Snakes play an important part in alchemy.  This stems in part from powerfully transformational properties of snakes' venom, and also from their ability to slough off their old skin.  Snakes lie immobile and dull for a time, and then emerge glistening new, giving an impression of immortality and eternal youth.

The ouroboros, a central symbol in alchemy, is a dragon or snake with its tail in its mouth.  The German organic chemist August Kekulé attributes his breakthrough on the ring structure of benzene to a dream about an ouroboros.  Arthur Koestler called it "probably the most important dream in history since Joseph's seven fat and seven lean cows".

This suggests that the alchemist has the ability to transmute the snake venom in his blood.

Hermes Trismegistus's phrase "As above, so below" stimulated alchemists to look for reflections of divinity in our world below.  It can be argued that this search of the natural and physical world led to the dawn of true science and the Englightenment. 

 A reference to William Blake's poem, Auguries of Innocence:

To see a world in a grain of sand,

And a heaven in a wild flower,

Hold infinity in the palm of your hand,

And eternity in an hour.