Hi, friends. First-timer here. I'd like to replace my shower valve, but I can't even get to it! The fixture is a Moen, likely seven or eight years old. Single handle turns it on, makes it hotter, turns it off. I got as far as pulling the decorative screw covering off and unscrewing the handle. Now there is some type of metal sleeve that is blocking my entry to the valve housing. I attached a photo, which I hope is visible. It is the round contraption on the right side of the screen. I had already removed the handle. It's that thingie on the right that won't budge. I showed the box-store guy, and he seemed to think gunk had built up and I needed to wiggle it up and down as hard as possible. Other than sore fingers and arms, no results.
One last thing: When I put it back together and took a shower, I heard water hammer when I shut the shower off, which I had never heard before. Did i cause more damage before from when I started? I thought I was cool doing the "hard" part, the valve, but maybe not!

Thanks in advance. You guys have a terrific forum here.

Sincerely,
Joe