The metal your frames are made of is an alloy that contains copper, which turns green when it oxidizes. It's the same with cheap gold plated jewelry that turns your skin green.
So the answer to your other question is "Yes!" The Statue of Liberty is also green because it's made of copper. It was metallic when it was first installed in 1886, but had turned fully green within 20 years.
Glasses frame is usually made of a type of copper called monel. Monel is a table metal and unreactive towards a lot of chemical. It is also skin friendly. The only downside is that it turns greenish when it gets rusty.
[edit] It's interesting that that article suggests a way to clean them--but the Costco optician just now said you can't really clean them. (They replaced them for no charge.)
Interesting point. I wash mine everyday. Water and diluted dish soap. The frames have titanium--but I bet the mounting posts on the silicon pads are the ones that are oxidizing!
I rarely wash my glasses with soap and water, maybe once every month or two. I DO regularly wipe them with a cloth, usually my tshirt. My nose pads have never turned green in 60 years of wearing glasses.
I've had this with cheap sunglasses that use copper screws to keep the pads on. The green is the copper oxidizing. Cleaning them would only make it worse as you're just exposing the copper to water.