My plants have mold on the top layer. The problem ...

Both these issues are due to over watering.

you need a better wet/dry cycle. If not allowing that top layer to dry before re-irrigating, you are causing the surface growth. Heavy soil should wait until top 1" is dry, then re-irrigate. IF fully saturating causes any negative effects, it is the fault of the soil constitution... more perlite or similar is needed in that context. a little growth early on happens, but with any mature plant, you should not be growing weird shit on the top soil. Seedlings can't handle a proper wet-dry cycle, but a more mature plant can. Growth on top is direclty caused by keeping it too wet for too long. gnats? Bacillus thurgenisis or something to that effect -- "Bti" - Genus species of a bacteria that has been used on the worlds food crops for a century. It's safe to use and can be used pre-emptively to avoid pretty much any pest that lays larvea in the soil. In north america, the brand is "mosquito bits" or "mosquito dunks" by summit. One is a top dress, one floats in a reservoir. First, search for the proper genus species name with google, as i spelled it wrong, lol, but it's close enough that it will correct you. Then, if you don't have that product in your area, search for the "B. thurgenisis (israelis)" name. the subspecies in in parantheses. I use that stuff early on and in my reservoir. If you get substrate that comes with larvea, which is not rare, this will prevent it from eating your roots. I use it the first month or 2 just to be safe whether i see a gnat or not.

No stress on the mold, it happens. You can spray some Neen Oil on it the next few weeks to get rid of it if you want. Neem oil can also help a bit with the gnats, but to really get them I would cover the soil with a bunch of diatomaceous earth (until you basically only see white), stick a few yellow sticky traps in the soil (to catch the adults), let the soil dry out (stick your finger in the soil to your second knuckle and when it all feels dry, then water), and when you water next use a mosquito dunk mixed into the water (and cover where you watered with diatomaceous earth again). Repeat these steps until the problems resolve (but I would recommend to continue the discipline of using your finger to check before watering to help prevent this in the future). Hope it helps!

Hey growmie, that white fuzz looks like harmless surface mold or mycelium — not dangerous on its own, but if your top layer stays too wet, it’ll invite those little substrate flies (fungus gnats). Here’s what I’d do: Let the top layer dry out harder between waterings. Gnats love moisture — make it dry, they’ll bounce. Mix the top layer a bit to break up that mold and maybe add some coarse sand or perlite on top. Makes it hard for them to lay eggs. Yellow sticky traps help a ton catching the adults. If it gets worse, do a watering or two with neem oil + BTi (like mosquito bits) ,that’ll wipe out the larvae safely. Shouldn’t be a big deal if you stay on top of it. Good luck ✌️🍀

1st price of advice wait till soil is really dry b4 watering everytime durring veg it’s ok if it takes 2 weeks between watering if soil is still wet. 2nd if your saying you have fugas knats in the soil you can mix 1part hydrogen peroxide and 5parts water and water all the way thru the soil it will kill all the offspring The yellow sticky traps work good aswell And mixing water with vinegar and dish soap and essential oils like mint and suck that buds don’t like There is a lot of products at plant stores you can check out at your local store Good luck