Ahhah. Bingo. correct.
Though what next? That grey piece still doesnt budge.
Should i have to remove the nut underneath and remove the entire assembly from above?
Looking at the pic, to me it looks like a 6 sided valve nut. Try using big channel lock or big cresent wrench see if it will unscrew. Use some wd 40 or pb blaster to help with the sediment.