One of the three moss items found over Lordran. The main use of Purple Moss Clump is to remove the affects of Poison.
You can use Purple Moss to lower the Poison status bars or wait until you have been affected to remove the ailment. It is highly recommended to have a few of these assigned to your Hotkey slots before heading into Poison Swamp at the bottom of Blighttown. There is also a small Poison water area in Lost Izalith.
The Undead Female Merchant will sell you as many as you like for 500 Souls each. You can farm all three moss types from the Demonic Foliage in Darkroot Garden. The Blowdart Snipers in Blighttown will often drop them when defeated.