After I got him from the pet store I feed him quite a lot of food, which consisted mainly of silkworms. He was not eating any "rough" bugs like roaches, crickets or superworms. However now that he looks much healthier, I have slowed down the amount of food I give him and he recently has begun to eat crickets again. Maybe the slimy stool is caused by the worms, since they contain a lot of moisture/water?