Polycythemia is a real disease in which the body produces too many red blood cells and is not, by itself fatal. Xenopolycythemia would be an alien (xeno- = foreign, alien) variation of the disease. It is revealed Dr. McCoy is suffering from xenopolycythmia.
Jon Lormer, who plays the Old Man, was also in The Return of the Archons (1967), and the original pilot, The Cage (1966).
The ancient Yonada text is based on Korean (Han-Gul-ma) alphabet.