Aki can disguise himself as a Hunter because he actually is one.
I'm sure Naikage knows about the whole "Inner Ki being is opposite normal self" thing, and probably would make allowances for Aki. Especially considering he knows what Omega does in terms of causing trouble for the Chosen Ones, I'm guessing he'd assume that InAki is going to be the Omega of the Chaos Hunters.
'Course, that doesn't take into account Aki's recent character development, but in all honesty Aki hasn't quite processed it either. It's gonna be interesting to see how this goes for him.
