This is diatom algae, caught in fuzz algae.
I usually avoid giving advice on things I'm not very familiar with. I have zero experience with dirt substrates, and have seen very little of this particular algae combo in my own tanks. But as Watercage contacted me and asked for my opinion, I'm giving it my best shot.
This stuff is common in a new tank. Usually goes away on its own after a few weeks.
If it persists longer than that, as it has in your case, I suspect an excess of dissolved organic material. I have seen it induced by severe overfeeding in several of my friends' tanks. Once they started feeding sensibly, it went away.
In your tank, it may be that the Miracle Grow Organic PS is poorly capped, and leaching a huge amount of organics. With EI dosing on top of that, your plants have a "much more than you can eat buffet", and are not able to effectively reduce the organic levels. Decreasing EI may help, but decreasing the organic levels is a far better strategy.
Increase frequency of water changes. Physically remove as much algae as you can each time. Chemical treatments are of no help alone - though this algae is easily killed, the killed algae just releases more organics into the water, and it will reappear with amazing rapidity.
The rate of leaching from the substrate will eventually decrease, but I don't know how fast. It may need to be recapped. Get a better opinion on this from someone familiar with this substrate setup.
I suspect you have too much light as well. The staghorn tells me that. While I don't think this is an issue with light alone, excess light certainly doesn't help. If CO2 can be gradually and safely increased, that too will help, though it won't solve the problem on its own.
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