White side of face on mobs

When I attack squids, zombies, and pigs one side of the block on their head turns white. I'm not quite sure if this happens with other mobs, it might happen as well. It happens when I am using a stone sword or my hands, may happen with other items as well.

Confirmed Mobs this happens to:
Skeletons
Squid
Zombie
Pig
Creeper