Honestly I'd say you just have overexposure issues here. IE, you've been looking at the piece so long that all you can perceive are minor flaws that most people aren't actually going to pick up on.
To me that arm definitely reads as metal, if you want that emphasized more then it might help to put on some kind of reflection or refraction to show it's a shiny reflective surface. However, that may interrupt the Simplicity of the piece as a whole.