The white marks are stress lines where the plastic was stretched as you were peeling it off. if you bend ABS it goes lighter on the bend. You can use a blow touch to fix that. just start far away like 100mm, the closer you are the quicker you must move it around.
I used to have to do it all the time, I now use a micrscopic layer of diluted glue on the textured PEI plate. They self release with no blooming/stress lines.