How to repair/replace this Delta stem?

[amtrak23]

We have a drip that has been escalating over the last few weeks. Popped off the handles to see what is there and found this setup, which I can not get apart any further. I have tried prying the gray plastic thing out via the little slots on either side, and clamping down on it an trying to pull it straight out with a decent amount of force. No movement at all. How to proceed?

[Terry]

Pull on the brass stem.
It looks to be a Delta faucet. Repair parts are easy to find for those.