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.
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?