This is likely a form of Trichoderma. It tends to be benign and can be found in most soils. It is usually always present. It becomes more visible when the humidity levels jump up.
I have personally experienced it this season. The change from summer to fall brought on very high humidity where I am. Many of my plants exhibited this to a more intense degree than what I see in your soil. If I were you I wouldn't feel alarmed quite yet.
I combated it successfully with making sure the top layer of my soil dried out sufficiently before watering the next time, adding mycorrhiza to the water, or adding a product like Trifecta Crop Control to my water regime.
