It looks like mould or biofilm.
How often are you feeding? Is there any food left 3 minutes after the fish are fed?
Can you go back to all your previous threads and update please.
Did you return all the neon tetras? They are unsuitable for your aquarium size and will be overstocking the tank. Overstocking causes nutrient imbalances which causes bacterial growths and problems such as you seeing.
Do you understand the cycling process?
Please post your water parameters as requested. pH, ammonia, nitrite and nitrate. An overstocked, uncycled tank will again cause nutrient imbalances and issues such as what you seeing.
I see a very poor substrate with a voids between individual stones. That is going to trap a lot of detritus like uneaten food, which will go mouldy and get stirred up whenever you disturb it. A sand substrate or a finer grade gravel would be easier to keep clean, you can see any uneaten food etc.
If you havent addressed the issues raised in previous posts it is a clear cause of what you are seeing now. Make sure your aquarium is stocked with suitable fish, cycle your tank, dont overfeed. Consider replacing the substrate with something more suitable for an aquarium, what you have looks a nightmare to maintain.