Maybe this is just one of his many mutant abilities. We'll never know until he actually decides to try picking up something else.

He returns her smile, eyes dropping to his hands for a moment. He doesn't really need to look at the strings, but it's just...interesting, making something up entirely out of the blue. This isn't any song of Greta's; he made sure of that before he started. After everything that's happened already, it's better to keep things light. "I always really did like playing," he confesses, "I don't mind that at all. But it's different doing it just for fun. And singing songs that aren't about murder."