That is hemp mat I got from Zooplus. It's beigy brown -probably looks green because it's next to the green shelf
It comes in a huge piece of mat and you can cut it to size. I first put a circle of it inside the rat sputnik for our last hammy to make it a bit cosier in winter and he loved it. I put it round the bars in the cage for our current hammy when he was a baby so he didn't feel too exposed and it just got left there. I guess it does provide a bit of insulation too! It's supposed to safe if chewed - haven't had problems with it being chewed anyway but it does start shedding after a while and looking tatty. But the piece you buy is so big it's easy to replace a small piece now and then. It's not really necessary.