I would suggest putting some of it in a container and taking it to the vet, I have had a rabbit who had the same and it turned out she had cancer :/ lucky we got it at an early stage as she is still with us now😊. Take him/her to the vet aswell and the vet can see what is happening rabbits are good at hiding illnesses because they are prey animals
Oh and it could just be night poops I can't remember what they are called but yeah
Those look like unformed cecotropes(if your bun is also producing normal round fecal poop). Is there any blood or mucous in it? Did you give your bun anything new within the last 24 hours, any medications, or does your bun get lots of sugary/starchy treats? Is your bun still eating, drinking, and behaving normally?