I would use a heatgun……
Blow it on HIGH in a small area on the surface of the surround and try to pry the plastic off the wall…..start at a corner/edge….MOST adhesives will release with heat……..
It was installed for a “reason”…..maybe the wall/bottom is rotted……hiding…..
Also the common instructions for gluing to tile include “sanding” your tiles to insure a good bond……..They may not be in any condition to reuse/use……….