Hey xin,
Hey I re created your issue inside of a new project targeting mobile platforms. When running on mobile preview the texture returns a green value.
First make sure your texture is assigned to your camera. This will allow the capture to be derived from what the camera is looking at.
Second, I tested this with an IOS 5s 8.0 and tried to launch it the project on the device. Under the Launch option in your main viewport there is a section called device manager. The IOS is greyed out with a message stating " The selected device does not support process snapshots."
At this time I do not believe scene captures are supported on the mobile platform. Additionally, cube maps are 6 times more expensive than a 2D capture.
So, as an answer to your question scene captures are not supported on mobile platforms.
Thanks,