Στιγμή (moment) and στίγμα (mark,spot, or speck) both originate from the same ancient Greek verb, στίζω (carve a mark). You can see how stigma and στίγμα are obviously related. :)

(Also, στίζω has the meaning of use punctuation marks, which is why στιγμή is another word for dot.It's not commonly used, this way though, not at all.)

I assure you that both Μία and Μια are accepted. In fact, all unaccented words are accepted; it is a policy of the course. So any error was not due to the accent.