You decide what you look like, including your height, your weight, the features of your face, the sound of your voice, the coloring, the length of your hair, sex and all other distinctive features. (Wayfinder Guide for Eberron)
The changeling's ability gives them the ability to look the way they want as long as you appear to be the same size and overall shape. And appearing as another race, even in one arm, is simply a matter of cosmetics (color, texture, features, etc.). In the absence of other restrictions, it's up to you to choose your appearance and nothing prevents different parts of your body from appearing different from each other.
The ability of the changeling does not require concentration because it is not a spell, but you return to your original form when you die.