Hint 1
You're looking for an element that's suggested by the High Puptess.
Hint 2
Elements are abbreviated as two letters. Do you notice any two letters on the High Puptess poster?
Hint 3
Look for a poster of the periodic table of the elements, on the walls of Hex & Co. This will help you decode the letters once you find them.
Hint 4 (Solution)
Silver (taken from "AG" on the High Puptess poster)