That looks like cyanobacteria to me. It's not an algae, but a photosynthesising bacterium and can be very hard to eradicate. The best chance is to suck it all out, do a large water change to get nitrate down and then black out the tank for three days (no peeking, no feeding). Then do another large water change and keep nitrates lower in future.
With the greatest of respect how do you suggest I ‘keep nitrate lower’ in the future?
I have tried almost everything I can to get nitrates down since I first cycled the tank and stocked it:
1 - I have returned 5 danios because I knew being overstocked was a contributor.
2 - I am doing twice weekly water changes to try and reduce nitrate levels (four buckets each time).
3 - I introduced live plants as I was told that this is a great way to lower nitrate levels in a tank.
I don’t know what else I can do and I believe the plant light has contributed to the blue green algae/Cyanobacteria becoming an issue as this only started happening a few weeks ago, certainly not before I introduced the live plants.
I’ve only ever run the light at 50% intensity, currently now 40% for a 9 hour cycle with 1 hour each for sunrise and sunset.
Is it ok not to feed the fish for 3 straight days?
The TNC root plugs do release nitrate so depending on how many you have in there compare to how much planting that may be part of the problem. Other than that, the other possibility is that you are overfeeding the fish and thus ending up with high nitrate from fish waste/waste food. Feed only as much as they can clear in under 2 minutes and just once a day.
I did leave the root plugs in once I'd taken the plants out but, as I've said before, high nitrates was an issue long before I introduced live plants.
I also have an 80 litre tank with a single male Betta and high nitrates are an issue in this tank too. I do two water changes in this tank per week, and there's only bogwood and gravel in there.
With regards to feeding all fish only get a small pinch of food, once per day, and it's all eaten well within two minutes, though the Betta is a slow eater so it does take him a little longer.
What I don't understand is that I thought, maybe foolishly, that because nitrates are so high in the tank, that the plants would thrive and the nitrate levels would plummet and all would be right in the world, but it hasn't turned out that way at all.