Thank you for posting! This is a strictly decorative balance cover. To remove it, you will have to pry it out with a screwdriver. Be careful though, as it may crack. Like I said, it is decorative, and does not affect the operation of the window or the balance.
I see you also may need help replacing the rest of your hardware. When the balance cover is removed, let us know what you'd like to replace and I'd be happy to help!