She could call you gramps if you wanted it, Tidus. Just saying.

"Um, I.. I don't think I've ever called someone by a nickname before.." That'd be the kind of thing one of her more outgoing friends back home may have done, but not Madoka herself. "And I try to not use Japanese honorifics with people who aren't Japanese here, since it might be confusing for them! So.. I think I'll just say Tidus."

That seems the most right thing to do here.

"I--I'm okay with nicknames for me though! As long as they're not too embarrassing.."