If you are going to use a dome you have to drain it every couple of days as humidity collects or you can prop one end up for a couple of hours a day to let it air out. That constantly wet warm environment with no air circulation is the ideal environment for fungus growth unless one uses sterile, soil-less seed starting mix. The little toadstools will die off quickly once you get some air in there and let it dry out a bit.

It can be difficult to find that line between just moist enough for germination and a humid swamp. ;-)

Dave