Yes, it is possible an allergy to the caseinate (milk protein) is causing the mucus. Might try a soy or egg-based formula instead... or an amino-acid based formula that has no food proteins in it, just free amino acids instead.
Chronic Lyme, Bart., Gluten & Sulfite Sensitivity, Many Food/Inhalant/Medication/Chemical Allergies & Intolerances, Asthma, Gut dysmotility & non-specific inflammation, Lupus, Osteoporosis, etc.; G-Tube
Meds: Flagyl, Domperidone, Zofran, Claritin, Andrographis & other herbs, homeopathy, supplements, etc.