Your name is Awesome.
1) Yea, if you keep calling yourself Mustache Spy, I think that other people will call you Mustache Spy as well.
A short nickname that you could get is the stache or spy. Or at least that's why I could come up with, I also have no idea what my nickname would be.
2) Just because you don't have a mustache doesn't mean that you can't name yourself Mustache Spy.
My username is JingIeBeIls and I don't make Christmas videos or wear anything Christmas related (Except for 2 videos, but it's not a standard thing on my channel)
You could even wear a fake mustache, just an idea lol