I do see the word "clean" being used a lot these days, even more so than "green." Here's my thoughts on the subject of why it's becoming a popular word (so popular it's being used in company names, government policies, and marketing materials).
1. Clean sounds better than green. Green can be directly connected to environmental activism and causes, and some people just don't want to go there. As Kermit the frog and Ray Charles would say, it ain't easy being green.
2. Cleaning up messes is a good thing. As things changed drastically over the past 100 years, America became a bit of a cesspool right along with many positive changes. If you think about the public information TV commercial from the early 1970s where the Native American chief cries after seeing all the piled up trash, thick smog, and filthy lakes and rivers, you'll see recognition of the troubling dilemma of creating the most powerful economy in the world and its dirty byproducts. So cleaning up messes is a good thing, whether they come from coal powered energy plants or smog spewing cars.
3. Clean is high tech. Most of the time, I see references to "clean" tied to the words "tech" or "technology." I think that's one of the factors that gets a lot of people excited about "clean" (and somewhat to "green"). It does require a lot of advanced technology innovations to make energy efficient and clean, to develop biofuels that work well without causing layers of problems, and producing effective, workable, long-range electric vehicles. It's very similar to the American dream that was realized when Neil Armstrong walked on the moon in 1969.