Viral Gastroenteritis: This is the most common cause of diarrhea in infants, characterized by the presence of mucus without blood, and the baby remains energetic. Viruses such as rotavirus, norovirus, and adenovirus are common culprits.

Bacterial Gastroenteritis: Although less common than viral causes, bacteria like E. coli, Salmonella, and Campylobacter can cause diarrhea with mucus. The absence of blood and the baby's energetic state make this less likely but still possible.
Toddler's Diarrhea: A condition seen in toddlers but can start as early as 6 months, characterized by chronic, non-bloody diarrhea with mucus, often without other symptoms.
Lactose Intolerance: Could be a consideration if the baby is being introduced to lactose-containing products or if there's a history of lactose intolerance in the family, though it's less common in infants.

Intussusception: Although rare, it's a condition where a part of the intestine slides into an adjacent part, which can cause intermittent abdominal pain, diarrhea, and sometimes mucus or blood in stool. It's a surgical emergency.
Infectious Causes Requiring Specific Treatment: Certain bacterial infections (e.g., Clostridioides difficile) or parasitic infections that might require specific antimicrobial therapy.
Hirschsprung's Disease: A congenital condition affecting the large intestine, leading to severe constipation or diarrhea, but it usually presents with other significant symptoms like failure to thrive.

Cystic Fibrosis: Can present with gastrointestinal symptoms including diarrhea due to pancreatic insufficiency, but it's associated with other systemic symptoms like respiratory issues.
Congenital Anomalies of the Gut: Conditions like intestinal malrotation or duplications can cause chronic or intermittent gastrointestinal symptoms, including diarrhea.
Immunodeficiency: Certain immunodeficiencies can predispose to chronic or recurrent infections, including gastrointestinal infections leading to diarrhea.