Green or white fuz spots on the
surface of the log:  This is a green
mold which usually forms on the log
in a commercial growing
environment.  If you see this scrape
it off and spray a very light mixture
of bleach/water all over the log.  
Green mold forms because of too
much humidity, so you may want to
cut back on the watering a little
and take the log out of the tent.
The white stuff you see are
spores dropping from the
mushrooms.
Tanins in the soaking water
from the sawdust in the logs.