First off, I'm not an expert on this type of stuff (which will be evident
in my post 8>))
But my first reaction is to make sure you don't have a moisture source
ABOVE the plastic. Is there any pipes that sweat there? Any leaks from a
sink or something? If you have a moisture barrier in place, ANY moisture
that gets in there will not be able to get out that easy. The fan and
vents should do this, but if it's coming in faster that it can evaporate,
you'll have problems.
As for the fugis, if you remove the moisture, you will most likely kill the
fungi. But I'm sure there are cleaners for this if you can't find the
source of the moisture.