Well, this will be interesting, because this quite clearly means that instead of cooperating with him, Mei is actually working against Arata's interests.
Wonder what the implication of nicknaming her "canary" is. The only reference I can think of is the phrase "canary in the coal mine," which doesn't seem applicable. So why is she that bird over any other songbird that might be kept in a cage?
She's simply using the terminology used by her new favorite textbook, of course. Now why the author of that chose this particular bird, that's quite another question...
We've seen before that she was bothered by it a bit.
Even in this very chapter. But she also knows they need the income. And, of course, there's not much she can say considering how she first met Mashiro, anyway.