For the time being, Erwin still tries to make it very clear when he's using the metal hand rather than the flesh one. He doesn't just do that for Levi, but for everyone. He knows it probably feels weird to be suddenly touched by something metal, and so does what he can to broadcast his intentions with his right hand. As times goes on, he might lose some of his self-consciousness about it, but for now, he lets people know.

Erwin looks up when Levi moans; he hadn't expected that, but definitely likes the feedback. Was Levi trying to suggest something, or did that just slip out? He watches Levi through his eyelashes for a few moments as he really digs into that sore spot, trying to open up the muscle and let out all the tension. Seems like it was accidental, but Erwin would be lying if he said he didn't like hearing it. He also only suggested massaging Levi's thigh because he thought it might be tight too; there was no subterfuge at play with the idea, but now... well, he's definitely paying closer attention than he was a few moments ago.

"All right." Erwin smiles, and digs at that sore spot again. "Finish your tea, and then we'll go to the bedroom and I can get the rest of your leg."

This might not lead to anything, and that would be fine. Worst case scenario, Erwin gets to run his hands over Levi's legs for awhile and admire the muscles on them, and Levi gets a leg massage. It's really a no-lose kind of situation for them both.