What is the "white" stuff on the soil?
A picture would surely be helpful but I'll give this a try. If you are growing indoors, white fungus on the soil often indicates that you may be over-watering. A houseplant that has been in the same pot for several years may show fertilizer minerals that have accumulated. It's a good indicator that it is time to repot with fresh potting mix. If you are talking about thread-like white strands in mulch or the top of outdoor soil, this is usually beneficial fungi. The following link goes to an article on white fungus on seed starting soils: https://www.gardeningknowhow.com/garden-how-to/propagation/seeds/preventing-white-fluffy-fungus-on-seed-starting-soil.htm