The head is just a Item drop like Gems and Such, the Moonstone Quest heads are droped by certain named spawns by the Parth farm. No worries, just sell it. It's like a Parth head that drops from the Parth's.
I killed siabras for a good part of level 16 and once a skirmisher's loot contained a "siabra skirmisher's head" it was worth 1000 cp to a merchant. It's not magic but of the 80 or so siabras I killed this was only dropped once. Im kinda wary of selling it since its rare. So if anyone has any info it would be appreciated. Thanks