It might be that he is eating greasy foods. Are you giving him fries or stuff like that? My dog ate some french fries and he had some greasy stools. You should just watch him for the next few days. If it is consistent then take him in.

This could be a variety of different things. As Sillylucy said above, it could be that your dog ate something greasy. One of the things I would take into account is how often he poops in the house. Is this normal behavior for him? If not, this could be a sign of a problem, such as him not being able to hold his bowel movement in and not feeling good. I would take your dog to the vet just to be safe. He may be having digestion issues as well. It never hurts to look into the issue. 🙂

Thanks for the advice guys, the only thing I could think of him getting near to greasy foods is probably eating the food I left on the table when I am not looking, or probably when I forget to put it up sometimes. And yeah, I'm just going to have to take him to the vet to see what could be the reason.

I can't really be able to know exactly what's wrong with your dog from the description of his poop. You should observe him for 24 hours and if it repeats again, you should take him to the vet. 

This definitely sounds like he got to some greasy food when you weren't looking which meant his poop has turned out the way you explained. It's not always easy to keep dogs away from food, especially not if you have to set it down to do something quick but I wouldn't worry too much to be honest unless he starts falling seriously ill with it. As long as it's not something that happens all the time, he should be fine. 

I don't take any chances with any of my dogs. I'm always very careful when it comes to what I allow them to eat. If they eat healthy, they won't fall ill very quick. I would suggest you get your dog to see the vet as soon as possible.