Hmmm....definitely worth checking out at the vet if you can't tell what it is.
Like RoxyJo'sMom said, it could be a string or something. I hate to be graphic (eww) but maybe if you put the poo in a baggie and move it around a little you might be able to tell better what it is?
I hope it's nothing
P.S. I had a cat once that ate from the garbage and he pooped the red casing from bologna, I thought he was DYING
Luckily my husband saw it for what it was and I was SO relieved
!