Hi,
there may be several factors causing nasal congestion. Mucus is usually a sign that the body is trying to rid itself of something that irritates it.
-Aircon-does baby sleep underneath or close? Mould in the aircon?
-Nutrition-are you breastfeeding or giving formula? Bf usually doesn't cause this problem, except if you consume something your baby doesn't tolerate. Dairy is notorious for causing mucus, not only for Asian kids.
Don't try to stop up the mucus production, as its existence shows that something needs to get out. Simple mild saline solution or drops of expressed breastmilk are very helpful to alleviate the discomfort.
For small babies with colds, very mild essential oils can be used (3 drops in a diffusor): Lavender, Eucalyptus, Thyme and Hyssop are appropriate.
Good luck!