There should have been a pre-slope bed put down before the liner. It should have have drained.

If it's mostly flat, it needs to come out and be done properly.

It needs to completely drain and not pool at this point. If it doesn't you are going to have a problem later.

Thank you this id helpful

I'll be honest, if your contractor didn't build the pre-slope before installing the liner, I'd be very concerned that he doesn't have a clue.

Do not move forward with a contractor who doesn't do things by the book. The finished product may look good, but it will leak and you will rip out the entire project and start over. There are lots of people who have posted on here who have leaky showers a year or so after install and it's a complete "rip out and start over".

Study up on the proper way to do it. There are lots of youtube videos of proper construction and what to look for. Take pictures all along the way.

Grout is not waterproof, so if the pre-slope is not correct, water will pool below the tile surface and eventually cause bio-growth and failure of the mastic and grout.

^^ Not to mention funny smells and possibly odd coloration on stone tile such as marble (because some of the stone is wet).

The big issue is that if a contractor doesn't know how to do this already - or read instructions - then he's probably the wrong person for the job. Red flags are waving.