I think it will depend on where you put the xGlow and what sort of tissue your body has in that location.
The aim of the glow powder is to remain in the epidermis, the topmost layer of living tissue, to maximize exposure and allow maximum illumination. The xGlow is much deeper than this, so ambient light charging will probably only be possible in direct sunlight, and only if installed in an area that is “thin skinned”. Some people have very little tissue on the back of their hands - they can see a lot of vasculature and any implants placed there are very visible. My hands aren’t like this at all, they hide my implants very well. YMMV.