It is lotion! I had same problem with wife's gs driver's seat bottom. She sure was embarrassed after we found out it was the lotion she uses on her legs

 

They are turning green due to sun damage. This will happen with certain leathers more than others. This is common with leather couches as well. There are many products to help you with this. Check out this link. http://www.leathercleaningrestorationforum.com/forum/showthread.php?8233-Restoring-a-Sun-Damaged-Leather-Sofa