I don't have any suggestions, just a similar problem!
I got rid of the top few inches of soil from a hibiscus I bought from Home Depot and replaced it with some organic potting soil, but every couple of days I scrape off some of the top layer because it's getting fuzzy... The water also smells terrible when it comes out of the plant, I should probably try completely re-potting it.
The soil I use is the Organic Choice Potting Mix from Miracle Gro. I'll get back to you after I replant it with just the Miracle Gro.
I had the same issue with the Organic Miracle Grow, which had like woodchips in it. I do cover with river rocks though too. I switched to an organic soil with perlite for better drainage. Basically soil needs to dry out and not be so wet all the time. Constant misting and high humidity you'll get mold and gnats if not ever drying out. Try to position the pots so all your runoff from drippers/misters does not hit the plants soil always and also position lights so they can help in drying out the plants through the day. Gnats are a pain especially in a the greehouse or lizard lounge .
Edit- look for any all natural organic soil, it will all mold and bring gnats if given the right environment though even the best of soil.