Hello my dear
I also have a cat , she also catches mice
Maybe it is not good to be happy about it. Shantideva teaches there are 3 kinds of karma and they have the same effect
committing an action yourself,
rejoicing in it,
inspiring others
If we rejoice in actions that later bring suffering we have the same negative karma as the one who committed them.
But it is good that you think about what to do, it shows your compassion and sense of responsibility for. Dharma is a path of compassion and wisdom.
I think I got some advice
Try to save the life of the mouse, of course. With compassion for the cat and the mouse who are in this situation out of ignorance and causes and conditions, they have no choice
Say a few o mani padme hum for the cat and the mouses - dedicate - may they and all other beings attain a higher rebirth meet dharma and attain enlightenment . Will help them , I am sure !!
do not worry ..
wish you good night my Dear
Tam