I went to my garage and I saw a fertilizer for like maple trees or something. I thought it would be okay for me to use it on my cannabis white widow that's flowering because I needed to feed my plant. It was 5.5.2. after I put it in the soil I added water and now mold has been growing on top and I scraped it off but then 2 days later some came back already.
It's just fertilizer formulated for a maple tree which probably likes acidic soil so the fertilizers chosen promote acidity. Cannabis specific nutes are a rip off. It's a plant and prefers a soil comparable to vegetables because it's not a shrub or tree. Any normal fertilizer works. It is probably salt residue; not mold.
Maybe get a pic? Helps a lot for when you have questions. It could be just santas beard or mycelium from fertilizer. Both are ok and go away once the fertilizers and soil settle down. Next time try espoma plant tone or tomato tone ferts. If u want a recipe, I got two you could try.