We are kind of trained to think mold is bad, but that may or may not be mold, but what it definitely is...is a fungus. Everytime I mix my own soil and cook it, I get a nice layer of that on the top and mix it all up before my grow. In the beginning i bugged out and i put hyrdrogen peroxide on it and it kills it all instantly without harming your soils ph and all that, does mess up the micro life in the soil tho. If you wanna grow using soil I highly recommend reading books on the soil food web, TLO growing, soil microbiology and all that. You wont regret it. On a side note, plants that small in pots that size shouldn't really require much feeding unless the soil is crap. I personally prefer mixing a well amended soil charging it up with lots of micro life and letting the plants do their thing, add some organic cal-mag (not the synthetic salty stuff) with water and thats it. My guess of why this happened when it did is the combination of the sugars and nutes prob killed of some microbes while creating a better environent for the fungus.