How can I seal a mouse hole with a refrigerator water line running through it?
Whooooaaaa!
Leaking refrigerator lines are the #1 cause for house damage.   They are usually on the main floor, in an expensive area of the house, and once they start leaking water just pours out.
So mouse issue is one thing.   A mouse issue (they bite EVERYTHING) next to a fridge water line - disaster waiting to happen.   Ticking time bomb.
You are going to do a 2 for 1.   Have less of a chance for water issues and get rid of that hole.
Step 1 - Buy a fridge shutoff valve

Step 2 - Take out drywall and install  valve
You need to cut out a small rectangle - 12x18&quot;ish.   Put Shutoff valve on a stud, attach it.   Put water line into the valve - test it.   Replace drywall over it.
Step 3 - Basic mud and tape and plug back line to fridge.
It's behind a fridge so you don't have to be perfect.
So hole is gone and your fridge line meets modern plumbing code.