It's called bladder sludge, or hypercalciuria. It's from excess calcium build up in the bladder. Some rabbits are prone to developing it. Because your rabbit is still excreting it, and it's not to the point your rabbit is straining to pee, has blood in the urine, has stopped peeing, or has stopped eating due to pain, you may be able to manage the issue without too much difficulty, by reducing dietary calcium some, as well as ensuring good fluid intake and mobility/exercise.
It's how I managed it for my bladder sludge prone buns. Rabbits do need some dietary calcium for good bone health, but for bladder sludge prone buns especially, I've found it best to not overfeed foods with an excess of calcium. For mine I fed minimal low calcium grass based pellets, low calcium veg, and free fed good quality grass hay that was medium coarse, and this managed the condition pretty well for them.
Though do make sure not to feed too coarse of a hay, as it's too devoid of nutrients and a rabbit could lose weight and have health issues on a coarse low nutrient hay when minimal pellets are also fed. So if pellets are minimized drastically, the grass hay needs to be mostly soft and green, with some crunchy stems. With a low to no pellet diet, I also offer a salt lick, as they can no longer get the necessary dietary sodium from their pellets.
https://wabbitwiki.com/wiki/Bladder_sludge_and_stones
Low calcium diet for rabbits
https://rabbitwelfare.co.uk/rabbit-health/further-reading/calcium-reading-case-study/(It's generally recommended not to feed iceburg lettuce due to health risks)
I would suggest getting your rabbit off of the alfalfa based young rabbit pellets quickly, and transitioned onto a grass based adult pet rabbit pellet. That alone should help some. Though you may also need to reduce the daily amount given as well. Usually transitioning pellet types needs to be done gradually over a couple of weeks minimum, to give a rabbits digestive microflora time to adapt to the new food, but because your rabbit is having bladder sludge with these pellets, and if you stick with oxbow but give the adult food, you should be able to transition the pellets more quickly due to a similar composition.
If your rabbit is a small to medium breed rabbit(not a large breed), I would reduce the amount of the young rabbit pellets to a max of a tablespoon per day, and add in a tablespoon of the adult pellets. This can be done in two daily feedings of 1/2 T of each. Then over the next week or two, phase out the young rabbit pellets completely so you are just feeding 2T of the adult pellets per day. You may find you need to reduce the amount even more. I wouldn't feed more than 2T a day, but what your rabbit needs to maintain good health may be different. I go off of the rabbit maintaining good body condition, good health, and making sure their urine has minimal calcium sediment. If the urine is not thick and creamy or pasty, and only has very little to no white chalky calcium sediment, that was a good calcium level for my rabbits.
I would also suggest weighing your rabbit at least once weekly, to ensure a healthy weight is maintained with these diet changes. If you find your rabbit can't maintain a healthy weight on reduced grass based pellets, free fed good quality green grass hay, and low calcium veg, you'll need to consult with your rabbit vet on how best to manage your rabbits condition and weight.
Monitoring your rabbits weight
I also feed low calcium/low oxalate veggies like romaine and cilantro. The spring mix you've been feeding, may be too high in calcium and contributing to the issue. Feeding low calcium veg also helps with fluid intake, which helps keep the bladder sediment flushed out.
https://rabbit.org/journal/3-5/calcium.html
You also have to consider the calcium content of your water. If you have hard water that's high in calcium, I know some people have used a low calcium filtered or bottled water to help reduce calcium amounts in their rabbits diet.
Then there is encouraging activity to help keep the sediment churned up in the bladder, to prevent it settling and building up in the bladder. Having things a rabbit can hop up and down from(at a safe height), is an activity I found works well for keeping those bladder contents from settling.
Sherwood seems to have a good urinary support tab that you could look into, though I can't say for sure how effective it is. But the owner reviews sound promising.
And as with any health concern, it's always adviseable to consult with a knowledgeable rabbit vet.
https://rabbit.org/veterinarians/
