The discoloration (usually white) is a stress mark from the pull on the plastic. Often, carefully applied heat will restore the color; try a heat gun or hair dryer.
Stubborn prints will usually release (or at least loosen) from the plate if you pop the plate with the print into the freezer for 10-15 minutes.
Use glue as a release agent. Liquid glue is best, but it is expensive. A glue stick works well, but it can be hard to apply evenly, especially on a textured plate.
I dissolve chunks of glue stick in IPA in a small spray bottle. I let it sit for a day or so, then if all the glue has dissolved, add more. When the solution is saturated, the chunks will stop dissolving, and the solution is ready to use. If you do not see glue chunks remaining in the bottle, add more - the solution may be too weak.
I put the plate in a cardboard box to control overspray, then spray and spread the solution over the entire plate, let it level itself, and dry. It will dry faster in the printer with a warm bed. One or two coats work very well on a textured plate. For a smooth plate, one heavy coat is good; two is better, but might create faint glue lines.
