Dorian took some of her hair and set it aside. He should have Transfigured the corpse into something discreet before the stench of decomposition filled the entire shop, but he forgot. Just before your arrival, he added Madame Jocasta's hair to the Polyjuice Potion and transformed into her. With his new appearance, Dorian told you everything that I wanted you to know, directed you to the Notts, and read your mind. Greengrass is truly the best among us in Legilimency. He had to investigate your memories because I needed to find out how much you knew about us and the real Scorpius.