Can I use pond liner for shower pan membrane. I know I can use glue for regular Liner but I want to use 1 if possible

Proper pan membrane at a plumbing supply how o Home improvements store is not very expensive... why not use the right stuff?

Anything else is likely to harden and crack over time... causing you serious problems.

I suppose that you can call the manufacturer of the pond liner and ask them, but I agree with Smoothy in that the PVC liner for a shower is so cheap it doesn't seem reasonable (in terms of potential problems) to seek alternatives... ;)

Mark