I don't think they would pay a lot of attention to each other. Endermen don't attack unless they are seen, and Slenderman doesn't have eyes. Impasse? If there was a confrontation, I'm still not sure what would happen. Slenderman at his most violent is a force to be reckoned with, but most of his actions seem to involve driving people insane and causing them to act for him/it. Endermen hit stuff and teleport. I still think the apple goes to Slendermane.