I have a new hosta flower bed with new black dirt that we brought in last year. We had a very wet summer last year and the dirt developed a white powdery mold or fungus all over the surface. I would cultivate and within a week or two it would reappear. I didn't lose any plants all season but it looked terrible. This spring I was putting in new plants and noticed that every hole I dug had a very thick web of root system under the surface of the dirt. I had to shake the dirt off of the roots to pack it in around the plants. I did find some of the fungus in the dirt. I haven't had any problem with unwanted weeds or plants coming up from these roots. I don't know if these two problems are related but no one in my area knows what I am describing.
Roots, as in tree or shrub roots will be coming from trees or shrubs in the vicinity. Otherwise the fungal growth is liable to be decomposing the organic matter in the material you bought. To hide it put a coarser textured mulch over the top. The only routinely important - or at least notorious - pathogenic fungus growth in garden beds is that produced by honey fungus. Search the web for Armillaria for more information.
