This is a clear case of an ID conflict.
The Ars Magica moonstone item uses the ID 20256, but this same ID is also used by Thermal Expansion for that "Destroy" item (that is probably used for something hidden in Thermal expansion). The Thermal expansion item is used because these are loaded later in the boot order (loading order is alphabetic over the jar/zip files as far as I've seen)and thus overwrite the previous Ars Magica item.
The solution to this is to modify the Ars Magica or the Thermal expansion config file so it uses a different ID for this specific item. Keep in mind that some mods require their itemID's to be sequential, so modifying one requires you to modify the other ID's in the sequence too(I don't know if this goes for Ars Magica and/or Thermal expansion though)