Mucous occurs from irritation to the digestive tract. This can occur after a bout of GI stasis, from intestinal parasites, pathogenic bacteria disrupting the microflora, cecal impaction, a new food introduced too quickly into the diet or too much consumed (especially high sugar/starch foods), if a rabbit is sensitive to a particular food, or when excess sugars and not enough fiber is consumed it can risk disrupting the balance of microflora and cause an overgrowth of harmful bacteria resulting in digestive upset.
Medirabbit: bacterial and mucoid enteritis, enterotoxemia
bunssb.org
With the mucous being orange colored and a lot of carrot being suddenly introduced into the diet just recently, I would suspect that was the cause of digestive irritation. Hopefully it was a temporary thing and it doesn't cause any further upset. But if there are no other diet changes and the mucous persists or worsens, I'd recommend contacting your knowledgeable rabbit vet right away. There are some instances of mucous in the poop that are considered an emergency situation.
Veterinary emergencies. What to look for, preparing for a visit to an emergency center, and links for rabbits.
wabbitwiki.com
For future food introduction, I'd recommend always introducing new foods into the diet gradually and one at a time, including ones that your bun has had in the past but not within the past week. A gradual introduction allows the gut microflora to adapt to processing the new food, and helps minimize the risks of digestive upset. Gradually introducing one food at a time also helps you to know if your rabbit is sensitive to a particular food.
[ Tablet users, try the "web" version by clicking on "web" at the bottom of this page for a better, user-friendly format . ]
rabbitsindoors.weebly.com