It's fine. If you just stir it up and keep it exposed to plenty of oxygen, the fuzzy mold growth will vanish. There was obviously some moisture being held by the mix and the dark, confined conditions allowed some fungal decomposition of the organic matter in the soil mix to occur.

Thanks for the response! It does also smell a bit off putting, probably due to the decomposition? Wouldn't say it smells rotten per se, but has a strange, off-sweet smell to it (in comparison to the other container I had). Is the smell some sort of bacteria? Could it damage my plants?

Also, any suggestions how to dry it out without making it a breading ground for fungus gnats or other bugs that like to make soil their home?

The smell is due to anaerobic conditions - no light, lack of oxygen and some moisture present. Again, completely harmless, if slightly off putting.

I keep my soil mixes in covered totes and they remain outside. And while the rain does not get to them, moisture can build up even with the lids firmly closed. I just dump it into my wheelbarrow and let it sit out in the sun for a couple of days until it dries out. I have never had an issue with bugs invading the soil while it was exposed. Nor have I ever had an issue with fungus gnats or other pests in my indoor plants.

Got it. Thanks so much for the advice! I'm a bit paranoid about the fungus gnats - had a pretty bad infestation about a year ago, and took forever to figure out how to get rid of them (mosquito bits was the only thing that worked).

Watering too frequently with a moisture retentive potting mix is the most encouraging environment for them. Using something like Miracle Gro will pretty much guarantee their appearance :-) Since beginning using the far superior 5-1-1 mix some 20+ years ago, there has been NO issues with these pests on any of my indoor plants.

id just sterilize it .. and make sure to let it cool off before use ... or store it as you will ...

use a 10% bleach solution to wash out the bin ...

ergo.. no gnats.. and nothing else ...

i once made a giant jiffy pop with a dollar store turkey pan and some alum foil ... and baked it in the oven ... i hope you know what a jiffy pop is ... made a couple holes in the top of the foil.. and when i saw steam.. i figure 212 degrees was close enough ... yeah it smelled a bit ... it was mad science.. so the smell was knowledge.. lol ...

this was always my caveat with making your own mix.. lol... you potentially end up with too much ...

ken

https://tipnut.com/sterilize-soil/

Ken, am am sure many who think that live microorganisms that work in containers like they do in the ground would debate you on that method. Ha