I think when I read the word moustache I say it like moo-stash in my head every time.  And I'm 31.
When was the first time you heard it prounouced like that?
I was 7 when i first heard my friend say it and that's how it first came to me.
I think that is awesome. It is cute that some quirks from childhood transfer into your adult life.
Now I say it to everyone and they think it's cute.
It seems like you already know that the word is pronounced with a shorter uh sound, so I don't think that there's anything to be embarrassed about.  As you say, it's just the way that you hear the word in your head, so it's fun to tell people about this quirk.