He needs to make his decision one way or the other before this Aziraphale person gets home, but the need for alacrity has to be balanced against the need to know the whole story. If she has a good reason for not wanting to go back to ROMAC, she's not going back to ROMAC. Period. Andrew knows too well how difficult it can be to leave their 'care.'

"I have," he replies, curious as to what fungus has to do with anything. It strikes him that she's not really speaking in the manner of a child of the age he was told to expect, and so he has to trust that it's not a non sequitur...but perhaps a metaphor? "Go on."