Mold in bagged potting soil
Hello, thank you for your question!
The white mold found in an newly opened bag of potting soil is most likely a Saprophyte that is decomposing the organic material in the soil. After the soil is opened and exposed to light and air it will be less inclined to produce the white material that you observed. While wearing a face mask you may scrape the white mold off for disposal. The soil should then be safe to use. If you wish you may return and exchange the bag for a fresher one at the garden center where you made your purchase.
I hope your seedlings do well!