“ I appreciate the invitation but I have a policy to not meet patients outside of work or take photos with patients.”
(And, probably, so does the hospital, at least for the meeting people outside work,)
If they need more, a “it makes me feel uncomfortable.” And walk away before they make it weird.
Whatever you do, don’t blame policies at the hospital because other staff probably are okay with it, and/or they’ll bitch at management who will respond with a “that’s not true….” Or something.