A person who conspicuously proclaims how environmentally conscious or green they are in order to achieve some measure of notice or status, or to show how much better they are than you.
It does not matter what the specific act of green-ness is, whether or not you support that act. What matters is the tone of the subject (sanctimonious,self aggrandizing).
Green nosers actually do as much harm as good for their cause by associating their behavior with the cause itself.
This does NOT mean that all environmentalists are green nosers, just those that use it as a tool for social leverage.
May also apply to organizations, institutions, and companies. Usually in reference to commercials or some kind of public relations green wash.
Noun: Sure, I'm glad he's into solar power, but does he have to be such a green noser? He just comes off as preachy and condescending.
Verb: Those green nosing jerks made all this fuss about Earth day, then left their biodegradable trash all over the place.