Did you check the underside of the other pot that you had sitting in there? That might give you a clue.
Looks a little bit too regular, round and uniform in size to be Perlite, to me.
I think I'd spoon it out of there, but put it in a jar or something so you can satisfy your curiosity - not to mention ours!
Btw, mosquito dunks and bits really shouldn't cure your fungus gnat problem, I don't think, unless you're putting them in the plant saucers where the water sits. They're meant to be put into standing water to kill mosquito larvae.
