I have this happen a couple of times. I'll see a corpse on the ground and it will have this greenish shimmer to it. I'm presuming it means I can speak to dead with it but nothing happens. This has happened on a corpse I killed ( I under stand that; I killed you and now you dont want to talk) but there are others that nothing occurs or it says the corpse has nothing to say. So I'm not sure if that greenish hue on them is a signal to speak with dead or not?