For a good long time, the first life on earth used C02. Its the original atmosphere of this planet, just like Mars.
It changed to O2 by bacteria and killed nearly all other live forms on the planet. The biggest extinction event we know of.
So, we know that happened. So other life forms can use C02 and evolve and find O2 to be toxic.
If the first life here was C02 based, and it was a freak event that bacteria would off gas O2, this would be a toxic planet to the majority of life of other worlds. Cause maybe that freak event didn't happen on their world and they stayed CO2 based.
Aliens might not recognize Green as a good color. You will notice that there are two colors of leaves that trees and plants use. One is green and the other is sort of Violet. They are very close as to spectral reflection, and there is speculation that the Sun changed up its light waves, at some point, to favor green plants. Other wise the Earth wouldn't be called "Green." It would be called the "Violet Earth."
There are SO many reasons, WE shouldn't be here at all.
To think some something else is out there, and is remotely possible to be able contact us, is just pure fantasy. And a way for people to sell books, and movie/TV scripts. ALL SCI-FI is political........ It always has been.
So there are UFOs. But they are not extraterrestrial aliens.