Like Preitler said, fly larvae, or some other external worm. Intestinal parasites look very different.
Medirabbit: pinworms
If that's fly larvae, I would recommend checking your bun over very thoroughly several times a day, for damp fur or poop stuck in the fur, and signs of fly eggs or maggots on the fur. You just don't want to risk
any chance of flystrike occurring.
Flystrike in Rabbits
If there are signs of fly eggs on your rabbit but absolutely no maggots, they need to be removed immediately and thoroughly, and the fur and skin cleaned and kept dry. If there are eggs and maggots on your rabbits fur and skin, get to the vet
immediately! If in doubt at all, get your rabbit checked immediately. Flystrike can be fatal to a rabbit in less than 24 hours.
https://rabbit.org/veterinarians/
If it's not on your rabbit at all and only in the litter box, it's not necessarily a vet issue (unless there are any concerns they're in your rabbits fur/skin). But I would recommend keeping the litter box as clean and dry as possible while there are flies present. And try and keep those flies swatted as much as possible. For a rabbit that has clean and dry fur with no wounds, there's very little risk of flystrike occurring. But keeping a clean and dry environment as much as is possible, can help minimize that risk.
