How old is your rabbit? If he’s very young, this could be an emergency (ie true diarrhoea).

If he’s not young, this could be mucus from the intestine, which would mean an irritated intestinal lining. I’d cut out the carrot pure, since carrots have a lot of sugar. When you’ve pure something, its easy to happen to eat too much of whatever it is that is pureed, so maybe this is a carrot “overdose”?

Do you see any normal poop? How does he seem (altertness, interest in food or treats, does he try to hide away, is he sutting hunched up or trying to press his belly against the floor (almost like a human trying to do push-ups)?

Looks more like sludge in the picture.

If he’s 10 months old he’s not a baby, so that’s great!

As LBJ says, this could perhaps be bladder sludge. Bladder sludge is thick and pasty and consists of calcium compounds.

Have you seen more stains like this?

Something to note about bladder sludge concerns is water consumption and exercise are both important to deter sludge, in addition to diet. I agree that from just one photo, it looks more like thicker urine than diarrhea or mucous.

Re: adding pictures, you can always upload a link from a website like imgr if you’re having trouble with the site!

You can absolutely switch him to adult pellets! Young rabbit pellets have more calcium and protein than an adult bun needs.

The flower mix sounds fine, what to avoid is mainly alfalfa, clover and very dark leafy greens such as kale and black kale.

It’s great that he gets a lot of exercise!

If he’s used to eating lots of hay, hay and water will be fine for a while. If heisnt a big hay eater, you should complement his diet with a small amount of pellets, just so he doesnt get too little nutritious food in him (rabbits that are unused to hay cant readily absorb all the nutrients and calories in hay).

Its very good to keep an eye on the urinary output. True sludge is like fine wet concrete. It will cake and crack when it dries. Excess urinary calcium will not cake, it dries to a fine white-ish powder that sticks to the bottom of the litterbox. It’s easily removed with vinegar and some water.

Young rabbits pellets have alfalfa, which has more calcium, so I’m guessing that is the source of the calcium! Alfalfa is also more rich and will cause darker poops. I think switching to timothy pellets and encouraging timothy hay eating will help both of these.

Calcium in the urine on its own is not something to worry too much about, you don’t need to worry unless you are seeing a really thick paste.

I would try changing his diet. As the others said, alfalfa pellets are unnecessary at his age and the excess calcium could lead to problems. Grass hay is best, but it doesn’t have to be timothy. Bunnies pee out extra calcium, so seeing some calcium in their urine isn’t usually a cause for concern. Bladder sludge, however, is a more serious issue.