You should be aware that it is perfectly normal for meat to oxidize and become grey in color. In this case, it is still safe, provided it has been stored properly.
I cannot be completely sure that this is the cause based on just your picture. If you have seen the oxidation-grey meat I am referring to and know this is not the same, then this is something different, and possibly dangerous (although Carey Gregory's bile hypothesis has some merit).
But nowadays supermarkets try to hide this process, afraid that their meat will look unappetizing to customers. They package meat in individual containers with low oxygen content (which also has the benefit of keeping it safer). If they still sell raw meat from the display, they only keep a small amount of display, preferably not pre-cut, and stacked tightly, so there is little surface exposure to oxygen. I've also heard that they use nitrites on the surface to prevent the grey-green tinge, but I am not completely sure this is true (it could be illegal, or the amount necessary to prevent the color change could be high enough for the meat to start feeling cured).
So, if you have never seen the grey meat I refer to, this could very well be an example of it. If
- you stored the meat properly
- the green surface is not slimy (at least, not slimier than the normal parts of the raw meat)
- the green surface doesn't smell unusual
- there is no texture difference, just the discoloration
chances are that this is not dangerous. You could still decide to be extra safe and discard it (nobody can prove that this is nothing bad), or you could accept the oxygenation explanation and eat it. Or maybe show it to a butcher or an older relative who was used to shopping meat before modern technology made oxidized meat surface a rare sight.