This is a good page in distinguishing passion and virtue: Stoics on the passions Passion in the Stoic literature refer to various types of intense feelings. Passions are usually regarded as a byproduct of an irrational mind. read more
Passion, as used by Stoics, refers specifically to negative emotions, as I understand it. In the paragraph which includes that quote, Epictetus is discussing the understanding of what is truly bad, and what is merely indifferent. read more
The word "stoic" has come to mean "unemotional" or indifferent to pain because Stoic ethics taught freedom from "passion" by following "reason". The Stoics did not seek to extinguish emotions; rather, they sought to transform them by a resolute "askēsis" that enables a person to develop clear judgment and inner calm. read more