Cap over the soil look's like fluorite or safe-t-sorb ?
If so,,then dust from these product's can leave residue on substrate and leaves.
Can't never get either product to rinse clear IME, but some rinsing to knock off the majority of the dust and sifting through hardware cloth can make sizeable difference in amount of particulates in/on the substrate which also reduces that which floats around with the flow from filtration or disturbance by you/me while moving or uprooting plant's.
Looks to be Emperor or penguin HOB filter and swishing the filter material from them around in some old aquarium water or dechlorinated water can help remove dirt and provide good flow.
Would keep water level in tank high enough to prevent water flow effect from the HOB filter but rather have just good surface ripple from the filter output.
Believe some type of consistent nutrient delivery scheme would be my aim.(in addition to nutrient's in soil).
In low tech,you might could get away with Flourish comprehensive at suggested dose twice a week.
Would maybe run lighting no more than six or eight hour's total for the day especially in NON CO2 injected tank.(a few more week's)
Seachem Excel would not be too expensive to use daily as directed for 20 gal tank ,and it can help plant's by providing more carbon than soil's alone.
Gotta use it near daily.
If the brown stuff is indeed diatom's which easily rub's off leaves/glass,rocks with your finger's,then as the tank matures ,the stuff goes away on it's own with nothing but patience.(some more stubborn than in other tanks)
Meanwhile,,I would add more plant's even if it meant removing a portion of them later due to crowding.
The relatively new soil should provide for the plants along with fish poo,fish food's,
But might take some time for root's to reach the soil dependent on how deep the cap is over the soil.
Is why I would hedge my bet with the afore mentioned Flourish comprehensive,or macro/micro package from Planted Aquarium Fertilizer - Home
All bet's are off however if the soil you are using is loaded with root tab's,osmocote like product's outside what soil might contain.
Hope some of this help's.
P.S., Would not use nitrate reducing pad or algae control but add more plant's as mentioned.
Nitrates will be used up by plant's which could lead to deficiency(depending on how well the pad work's) and algae has a tough time in heavily planted tank where plant's are actively using same nutrient's as algae for growth.