Certified GKH Gardening Expert
It is hard to say. If you are using wild soil to germinate , then it is best to pasteurize the soil first at 180F degrees or just a little above for 3 hours prior to planting. It looks like one of the many leaf rotting fungi, and they probably won't bother your seedlings. If it is going after more than leaves, and rotting wood then you may want to start over, as they have a chance at eating your seedlings. It just depends on whether the fungus prefers living or dead matter.
Here is an article that will help you to pasteurize your soil: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/sterilizing-soil.htm