This seems to be a common issue, discussed in the Grand Cali section of the forum.
Hi, I ran into a problem with GC600 (early 2020). The waterpump keeps running even without any tap open. Checked the fuse - It's ok. When I put I back, it starts running again. Check the outdoor shower, it is turned off. I opened the water tank; the pump is (logically) got. But no symptoms of...
vwcaliforniaclub.com
The main problem seems to be water running back down the tap and causing issues with the micro switch, which then randomly activates causing the pump to run.
I would suggest you request a new tap rather than a pump. How old is your GC?
Good luck