As mentioned in varro's answer, there are rules about the placement of accent in Greek words. The Wikipedia article on Ancient Greek accent summarizes a pertinent rule as follows:
If the ultima is short, accent can be on one of the last three syllables: the antepenult, penult, or ultima. When the antepenult is accented, it must have an acute whether it is long or short. When the penult is accented, it must have an acute if it is short, but a circumflex if it is long.
If the ultima is long, accent can be on one of the last two syllables: the penult or ultima. The penult must have an acute accent, not a circumflex. When a one-mora vowel in the ultima changes to a two-mora vowel, or an ending adds a syllable, the accent moves forward in the word.
The Wikipedia article says "There are a few exceptions to the ultima length rule" (for example, in words where the last syllable is derived from an enclitic) but none of the examples it gives are words with penult circumflex accent and a long ultimate vowel. So, as suggested in ktm5124's original post, the circumflex accent on the penultimate syllable of "μυῖα" seems to be strong evidence that the vowel in the ultimate syllable was short, not long.
According to the Wikipedia article on Ionic Greek, the following sound changes applied differently to the Proto-Greek long vowel *ā in different dialects:
Proto-Greek ā > Ionic ē; in Doric, Aeolic, ā remains; in Attic, ā after e, i, r, but ē elsewhere
Wiktionary and Liddell & Scott list "γενεή" as an Ionic variant of "γενεά". If we assume the two forms are fully cognate, and that nothing else is going on that affects the vowel length, that implies that the Attic form has a long vowel.