Bucky starts walking again, but with a slower pace and shorter stride than his usual, out of deference for the differences in their height.

"I'm human." He glances at her again, out of the corner of his eye. Not many people would have apologized for such a question; at least, not many that he's met. It's curious. "Mostly human. I think you can guess which part of me isn't."

It's a poor attempt at a joke maybe, but there's no hiding his arm, at least not until (and unless) he chooses to look into new clothing. But that's certainly not an immediate concern and as such he's put it aside.