Can be normal or can be a disease. Need images to know for certain.

In what state do you live?

I've seen situations that fit your description in gardens where the mulch has been allowed to stagnate for a long time. This occurs when layers of mulch are applied over the top of older mulch...over and over again. A mycelium growth develops on the mulch (white, fuzzy) that becomes hydrophobic; soil underneath this kind of stuff can actually begin to turn anaerobic.

BMPs (best management practices) should include that the old mulch be physically mixed with new at each application. At the very least, the old mulch should be stirred around with a rake before a new layer is applied.

This is merely a SUGGESTION based on a very limited description. Would it be possible for you to provide some good, close-up images for us to see of the white stuff?