When storing substrates in conditions with high temperatures and high humidity it is possible that saprophytic fungus can proliferate, but it is not harmful to you or your plants.
The saprophytic fungi live and feed on decaying organic matter, they do not live on or damage living plants. There is not any adverse effect of saprophytic fungi except that is visually unappealing.
To get rid of them, simply open the bag, allow it to aerate and mix thoroughly, this will cause the fungus to collapse.