Sorry - didn't see the other posts. That's a piece of mussel shell - I just looked up Ziwi Peak rabbit and lamb canned cat food. Mussells are one of the ingredients. Some of the shell most likely gets mixed in when they manufacture the food. Don't think mussel shells (or bones) are good for his poor intestines right now. Are you in New Zealand?
The limited ingredients in the Ziwi are good, but I would try Blue Buffalo cat food for sensitive stomach. Make sure he has fresh water to drink every day also. I know I keep touting Blue Buffalo, but I've had good results with it for years - much better than any other brand I've tried.
IBD sometimes causes bloody stools. Or, the mussell shells are causing irritation.
As for the long stools, I'm not an expert, but I know IBD or lymphoma will cause his intestine walls to become thick. If they're thick, wouldn't that mean the opening for food to pass through is narrower? If he's not on a steroid to reduce the inflammation (prednosolone) already, you should talk to your vet about putting him on asap.