This problem could have two causes:

1. The child chewed on the nipple, causing a rupture. Afterwards, the nipple wasn't cleaned properly, what caused mould to grow
2. If the pacifier already had mould on it whilst it was in the packaging, it is probably due to bad storage in a humid environment for a very long time.

This problem has two possible solutions:

1. Sterilize the pacifier by boiling it in hot water. Dry the pacifier with a clean towel. Make sure not to forget to dry under the silicone rim of the pacifier.
2. A good and easy way to clean your pacifiers are the antibacterial paci-wipes by Nûby™ Citroganix™. This all natural product is antibacterial and prevents mould from taking hold. Fortunately, the formation of mould is a rarity with the new generation of Nûby™-pacifiers.
Do you have an old pacifier and are you having problems with keeping it clean? Then it is time to get yourself a new one.