I've read that a few people have had success with this polishing kit for taking care of cloudy mylar:
https://www.amazon.com/gp/product/B001AIZ5HY
I won't remove mylar unless absolutely necessary--it's too much of a pain, and carries some risk of lifting paint. It did its job in protecting the playfield, and it really doesn't affect play all that significantly. Clean it and wax it, and balls travel over it fine.
I also add mylar to protect from wear when I'm done shopping out a playfield. Most of the older games I work on don't have the benefit of those new protectors anyway.
Marion, MA
