By Homegardens
Canada 
What is growing in this dirt sowed onion seeds a week ago place a dome over top put on a heat mat removed the dome and found this
6 Jan, 2018
Answers
Agree, its mycelium from a fungus of some kind - all soil (if you didn't use sterilized potting soil) contains fungal spores and mycelium.
6 Jan, 2018
Agree with Stera - but don't be alarmed. It's still usable. Once you remove the dome, the extra humidity will disappear & so will the mycelium. It's like when you see mushrooms after the rain.
7 Jan, 2018
Its a fungus enjoying the warm damp atmosphere.
6 Jan, 2018