Sorry you have to put up with both "henna gaijin" and "otaku" remarks. I wish I knew what it was I was doing to avoid both labels despite being quite the oddball and a bit geeky...maybe they're just scared to say it to my face because they think the intimidating American girl's going to eat them or something
In the end, you just have to pick your battles. If your friends calling you "otaku" in jest bugs you, tell them or it'll just continue. No one can or will understand what kind of person you are just from one encounter - I mean, even I'm working from guesswork here - so while it hurts sometimes to just grin and bear it, sometimes that's just what you have to do in the face of being stuck in passing with a not-so-savory label.