I know exactly what you are talking about. Three of my dogs have that all the time when they switch back to raw. Some have it coating their entire stool and some have it coating only part. (We don't have whipworm in Japan, so I know it isn't that.)
Wait another week and see if it continues after that. If it lasts, it could be from worms or parasites, but if it clears up on its own, I wouldn't worry. I've always assumed it's the intestines clearing themselves of gunk, like people report having happen after they go on those 1-week colonic cleanses in Thailand.
Keep us posted!