Oh, it looks like mud or something. Maybe that's from the sand I don't know. Since it's coco you might want to water it more evenly and not just in the center. The bti or gnats wouldn't be the problem. My guess is something with the medium.It is coco/ perlite 70/30 with play sand topping-
And i do use bti occasionally- it just seems that the gnats show up inside as soon as it gets cold here. I don’t have a gnat problem- just a preventative measure