Furry growth on blood fish and bone

I sprinkled a little BFB on the tops of the tomato pots and watered in and today when I went to the green house it was furry on the top so I scraped it off. Perhaps I should have got a photo first but has anyone any ideas why and what it was?

As long as it's not harmful to the plants that's the main thing, the ones outside that have had a better watering plus it's rained on them were fine