I can remember Harry at a very young age, maybe 18 or 20 weeks with poops like this, I can also remember taking him to a dog park for play with other pups and having to get a bucket of water to wash away his poop because I couldn't pick it up - too watery. I pondered this for such a long time and sought advice from friends and then my local vet, they all suggested a food allergy.
So, I decided to look at chicken first as many people I spoke to advised their dog is allergic to chicken, I gave him the Canidae fish based one and no chicken at all, his poop improved greatly. All was well for a while and then he started pooping badly again, he lost a huge amount of weight and at 2 1/2 years old he looked and weighed the same as a 1 year old.
I took him to a vet experienced with GSDs and she immediately diagnosed him with EPI - his is doing brilliantly now with enzyme replacement and treatment for SIBO (using Tylan) SID (SIBO) - EPI * Exocrine Pancreatic Insufficiency
Just keep an eye on him for further problems.