This could have several different meanings depending on the context. The most obvious meaning these days would be asking how environmentally aware or active a person is. The term "green" has been applied quite a bit in recent years to describe environmental awareness, as it is easily associated with a strong environment (trees, grass, bushes, etc are all green). Such as "green" building design, which may mean it's built to conserve energy.

If this doesn't make sense, you may want to include the context of how you heard it to better pinpoint the meaning.