Without seeing the mold, it is difficult to say for certain, but fungal organisms, including mycorrhizal fungi, can produce a white mold-like substance on soil called mycelium. It is part of their normal growth cycle and if that's what this was, it should not have been harmful to the seedlings.

Various other, less beneficial fungal organisms can produce a similar mold like growth and these can be a problem. If your seedlings failed to thrive or died, I'd suspect something like this instead.

Mycorrhizae is not an essential element to successful seed starting. If you wanted to use it, I would wait until your seedlings are ready to be potted up.

I believe many of the mycorrhizae formulations contain significant amounts of nutrients to foster the growth of the fungus, and to maintain it in the packaging before that. Those nutrients may be responsible for helping other molds to thrive on the surface of the seedling trays. You can have similar problems if using water with fertilizer to moisten the trays when planting the seeds.
Renais

In an article in a recent issue of Fine Gardening magazine Jeff Gillman, PhD in soil science wrote that what is sold for "mycorrhiza fungi" is a waste of money because even if what you buy has the fungi you need to form that symbiotic relationship the shipping of the product probably will kill those fungi. Just for information the people at Espoma dispute what Gillman wrote, but then they have a product to sell.
Mycorrhiza, myco = fungus and rhiza = roots, refers to a relationships some species of fungus form with some plant roots. The fungi that form that beneficial relationship with trees will not do the same thing with flowers or vegetables.

That "mold" formed because of some fungi but on the surface not likely from a fungus that would form that symbiotic relationship with the plants roots.

Thanks for the feedback. We will be monitoring the seedling flats for any signs of fungus/growth.

I always thought seed sprouting medium does not need much in the way of nutrients or anything else since the seed carries everything with it to get the sprout going initially.

I'm a huge skeptic when it comes to companies selling additions to soil (other than the big 3 NPK perhaps) because so much of it is larcenous BS. Snake oil. Packaged fungi.

I don't understand how any seed sprouts better in amended dirt over a sterile medium. Fungus propagation and sprouting seeds are two things, to my little pea brain, just asking for a fight. The wee little babies are fragile and tender, so long live the microwave!

Seeds will germinate and grow until they produce those first true leaves without any nutrients in the growing media. Once those seedling have those first true leaves they do need some nutrients but too much is as bad as not enough.
Theoretically, seed starting media with added nutrients have time delayed nutrients that will be slowly released as the plants growing in that media needs them.
In all of the years I started my own seeds I did not use, and saw no need to use, that expensive growing media.