7 hours may be enough to trap the mouse, but it is not enough to trap the mouse and then fly the plane from China back to Helsinki to pick you up. That' s most probably why the flight was cancelled.

Whether are eligible for compensation depends on whose responsibility it is. I would say this case is not clear cut. On the one hand, if a passenger claims to see a mouse and it turns out there were no mice, you could argue it is the passenger's fault for lying or for not seeing well, and the airline is just a victim. On the other hand, if there was a mouse introduced by the airline catering or ground handling contractors, then it is the airline's fault.