diassambling this faucet

one of my HW sink faucets is leaking.
digging in to see what needs replacing I am stumped at Step 2 - getting access to the guts of the spindle.

the grey plastic piece seems to have flats on it which would suggest that I would use a wrench to unscrew and remove it to pull out the brass post and whatever is below it and has failed. (Cartridge? O-rings? Packing? Rubber washer?)

but it doesn't budge. or at least not without the entire fixture spinning on the sink deck.

does this look familiar to anyone?

many thanks

Peter