Again, back to the design of the mouse. If the mouse's flash is recognizable as a flash drive, it's probably going to be infected.
I have a cheap printer that has some on board memory that shows up like this. The printer uses it as a cache, but I could easily load that memory with whatever I wanted. I doubt that the mouse would have such a design, but probably there are a few out in the world built like that. Do you have one, I dunno, I would guess not.