I agree with @Crispii , it does look like cyanobacteria. Is it slimy to the touch? I have never heard of any fish eating this.
Cyanobacteria is due to organics in the presence of light. The water becoming hazy green is unicellular algae reproducing and clouding the water greenish, though the cyano could tint it too. Caused by the same thing, high organics (nutrients) and light. The organics is the main issue, more than the light, so the only way to get rid of this is reducing organics.
You are changing water every three days so that will help. Vacuum as much of this out each time as you can. Clean well into the substrate to remove organics. Keep the filter well cleaned, also reducing organics (the brown gunk is organics). Do not over feed. Do not have too many fish or too large a fish for the tank. All of this increases organics.
Some will advise reducing light with blackouts but this is only a bandaid. If the underlying organics are not cleaned up it will come back.