I was just about to go to bed when I did a spot-clean of Toki’s litter box. There were about 50 total poops in there, and I noticed that one side of one of the poops had some yellow stuff on it. The rest of the poops were fine from what I could tell, but I didn’t inspect them all individually, I just sifted them around in the litter scoop. Toki is eating, drinking, acting just the same as normal. (He is 2 and a half years old.) I probably wouldn’t have even noticed it normally, but it just happened to catch my eye.
What could be causing this? Should I go to the vet? 
Edit: here are some pictures

Edit: After finding some more of this on one of Toki’s poops this morning, I’m not even sure if this is mucus. At first I said it was mucus as it looks a bit like the mucus from the ‘Bunny Poop Guide’ thread, but now I’m not even sure what it is. I found some of it on his litter pellets as well, but it is in very very small quantities and therefore hard to tell what it is. Any ideas of what it might be?