That's exactly what it is: stained sponge rock. I do see some other white-ish stuff in photo #2. It could be mold but looks more like leaching minerals or crushed sponge rock.
Fungus gnats only stay in soil kept too wet. As you are growing succulents, fungus gnats should never be a problem. If your soil is wet enough to attract gnats, your succulents do not have a long life expectancy.