Greycap is a poison. It is made from toadstool.
A Clash of Kings
Tyrion Lannister sees powdered greycap in Grand Maester Pycelle's collection of medicines and poisons.
A Storm of Swords
During Tyrion Lannister's trial for the death of King Joffrey Baratheon, Grand Maester Pycelle claims that Tyrion stole a sample of greycap as well as other drugs from Pycelle's collection.