Are you planning to use the same soil medium you used when transplant ? - which may very will still prohibit a issue with the bugs in the medium. Round 2 is either try steralizing your medium...and or medium that is clean. Pickup up a bale of BX just last week due to concerns about seed starting bugs

To rid yourself of the mold -- and if it is grey it is not moss -- you can drench/water with Physan. Should be easy to find at a hydroponics store.

Can't advise on the wetness of the mat as I have no practical experience with them. Does sound like it may be too wet though.

Fungus gnat larvae can be killed off by watering with water inoculated with bacteria Bacillus thuringiensis subspecies israelensis ... commonly available in "mosquito dunks" sold at any BBS. One can break one of the 'donuts' in quarters, put it in a gallon jug to sit overnight, agitate the water prior to use, then water the plants. The donut chunk will float and will not dissolve completely -- that is normal. Do not throw the chunk away as it can be reused until it completely falls apart. NOTE: This will not create immediate results and will not affect the adult gnats.

Update: I removed the entire top layer of the what I described as moss. I've since left a window (about three feet away) open a crack for circulation. But after several days it is now reforming. The medium I used was Jiffy natural & organic seed starter, I'll shop for the physan and try using that. . As far as gnats I haven't seen any since my original post so perhaps it was just dust particles or something floating around.

If you have diacam. earth, you could sprinkle that. But if you are spending money to buy something, personally I would just put the $$ towards a clean medium

I have DE I can try some of that. When you say clean medium not sure what you mean because mine was a brand new bag just opened so should't that be clean. As mentioned I used Jiffy natural & organic seed starter.

Jiffy, MG Potting Soil, lotsa big box stores - they have eggs/bugs in them. Some worse than others brandwise. Farfad might be at our local nursery or Pro Mix/ Sungro from Horticultural places have soil less medium that don't have said issue.

Lesson learned I got Jiffy at Walmart, thanks. What does the DE do for moss I have it on hand to kill some crawling insects.

Try sprinkling cinnamon on the mold. Cinnamon has antifungal properties and it will kill the mold but it will keep returning as long as the medium is too wet. Camomile tea will also kill damp off fungus. I'd ditch the mat and replant your plants in containers. You risk losing your plants with overly wet soil all the time. A bit of dish soap--just a small squirt in a watering can with warm water will kill the fungus gnat larvae and any other bugs in your soil. Wet your soil with it --not excessively--and then fill your containers and plant

The DE is sharp. When they pass through it...it kills it.