I'm afraid to twist or yank something I'm not supposed to and break something. I've tried pulling up the little tab on top of the grey piece but it feels like it will break before it detaches anything.
The grey tab on top, and probably one on the bottom, open to disassemble the cartridge. To remove the entire cartridge, since you've removed the large nut, just grab the entire grey base, twist back and forth while pulling out. There are some "o"rings that seal the cartridge to the valve base and they can stick. Some manufacturers have a small slit or groove in the cartridge where you can use a slotted scewdriver to help pry the cartridge out.