"Walk the green mile" is an English Idiom. If someone or something that is walking the green mile, it means it is heading towards the inevitable. Anecdotal sources suggest that it came from the death row of a Louisiana prison, in which the condemned took their final walk down a hallway of green linoleum.
There's a film in 1999 called The Green Mile (adapted from the novel of Stephen King) which features many supernatural things that happen in the Death Row of a Louisiana prison in the middle of the Depression in 1935.
The Green Mile is the nickname given to the Cold Mountain Louisiana State Penitentiary's Death Row, which has a floor with a faded green colour where the prisoner takes his dead man's walk.
Learning a language is complicated and idioms are one of the more interesting topics. Check out some things to think about when talking about idioms in classes at Busyteacher.org.