What are some reasons for an infant to have mucousy stool? He's 3 1/2 months old and I've only noticed it once so far, but its making me nervous!

Depends. Once only may be of little or no consequence, particularly if there are no other symptoms. If it becomes a repeated observation, get it checked out.
Mucus is produced in. the upper respiratory tract. Infants do not cough it up or spit it out, they swallowi it. It may be visible in their stools. Customize healthychildren.org for your baby's age to see tips on development, sleep consolidation, nutrition & health. Enjoy!