-
The letter's name is spelled μυ (μυ υψιλον), which is pronounced /mee/ in Modern Greek and was probably pronounced something like /my:/ ('y' like a long version of the 'y' in 'dysfunctional') in Ancient Greek, as it is today in German (where it is spelled 'My'). Naturally, in English, it would be pronounced more like /mu:/ or /myu:/.
Traditionally, the letter ypsilon (υ) was transcribed as 'y' (which in German is still called Ypsilon), but more recently has rather been transcribed as 'u'.
You now need to change your username to μυμυcarthy, of course.
I've always heard it pronounced mew, written mu/μ. Though apparently my is used also (never seen that myself until around 30 seconds ago!)