Yeah this is a starter algae which is still there because your tank is a low e tank and you do not have enough house keepers (algae eaters).
Small foreground plants usually get affected by this algae with soils like Amazonia in the first few weeks. Not all the foreground plants but HC is one of them for sure.
A have a few thoughts on it.
People use Amazonia often, but this soil is made for Nature Aquariums. With its fast growing plant masses. It means a lot of light (9-10 hours per day), CO2 (a lot), fertilizers (daily) and tons of plants for start. That was the concept for this soil from the beginning.
In this condition this substrate perform really well. But if you put this into a low tech environment you will face with issues usually. Nothing major which you can't fix, but a could be a nightmare for a beginner. I also have many shrimp breeder friends who use it for years now in their shrimp rack, but they are not growing HC
Why you put in a soil to a tank where you really not growing plants so fast? Just because people saying a lot of good things about it or because of the soil has good reputation in the aquascpaing and shrimp scene does not mean this is the best way to go.
This could be, but need more work, and probably better plant selection which is aligned to your equipment.
Your plants looks weak compare to the 4/5 weeks timeframe. The HC gets less valuable light, no CO and not sure this was a potted or a jelly plant, but maybe the fertilizers are not aligned to the tank.
Hard water soft water?
Lot of yellow leaf, less green ones means no CO2 for HC usually.
This brown algae will die off or you can put in more maintenance work with water changes to eliminate it in 1-3 weeks, but usually enough if you put in a lot of shrimps which eat it up in a few days.
Since this is a small tank no amano shrimps pls
Or just smaller ones. Better to throw in 30 tigers and they will clean it up quickly.
This algae block the light so if you have a lot of it the HC will strugle.
So i see 2 issues here:
- Soil is not aligned with the tank parameters
- Bad plant selection (very CO2 demanding one) there are much easier alternatives
For shrimps you not mentioned which one, but Crystals, Taiwans can live nicely on Malaya for example which has lower pH and no ferts in it.
For Amazonia better to have good filtration, good maintenance routine, and give energy to your plants to use up the fertilizers from the soil.
Or just use less demanding plants with more maintenance.
No worry even the bad ass scapers face with this algae many times.