That looks like a dry white, chalky texure so I'm assuming the rabbit has too much calcium in his diet so I would get a sample and let the vets check it out and diagnose him.
Possibly just the calcium in the pee, but hopefully someone with more experience will be able to tell for sure. In the meantime I'll find you a helpful video.
It's calcium sediment in the urine, which can be normal. If the white stuff is just a thin layer and chalky, that's normal. If it is thick, creamy, or gritty, that indicates bladder sludge and you will need to get your bun seen by a rabbit savvy vet right away, as it can be painful for them and lead to other health problems developing.