Hello and welcome to Petco Pet Education Center, formerly Petcoach. It looks like Starky got into either some food that was yellow and bean-like, or that he ate something that is fibrous, as it almost looks like wool. Without being able to feel it to see how it breaks apart it is hard to tell from the picture. You could bring the feces to the vet to have them look at it, however if Starky is doing well, I would just monitor his feces for now, and make sure he isn't vomiting. Best of luck with Starky!