Harry's statement leads Alex to want to ask two questions. The first is obviously having to do with the fact that he called it a 'creature' not something like a polar bear or whatever. To Alex herself, calling something a 'creature' means that you have no idea what the hell it was.

But the second question is what Alex chooses to go with because he's her friend not a story. So Alex just gives his hand a small squeeze before she asks softly. "Because you were a doctor?" It makes sense to her given everything. If you were out on an ice pack she would want someone who could help heal people too.