This is a moisture mold due to lack of UV light (from lack of Sun), and it is harmless to the seedling.
Once the seeds start to germinate you should remove the half globe used to cover the growing pot and place in a well lit area so that it receives lots of sun light. The edges will dry out and any mold will disappear.
If you have lots of mold you can:
Use a mixture Hydrogen Peroxide dilted with water (only 3% Hydrogen Peroxide). Hydrogen Peroxide stimulates oxygenation in the
soil and also encourages increased root growth. This is the best method.
Or use vinegar diluted with water in a spray bottle (10:1 ratio of
Water:Vinegar). You don't need to use much.
Another option is to use ultraviolet lights to dry out the pot and give it a heavy dose of UV light.