Aquariums and Fish Keeping

Looks like you have cyanobacteria, which is a photosynthetic bacteria (sorta like algae but not exactly since its not a plant). So much like others have stated, imbalance of nutrients, photoperiod, and water flow will help prevent it from continuing to be an issue.

Manual removal is going to be important, esp cleaning the substrate with a vacuum tube to get any excess organic matter out. If possible, add in extra flow through a water pump or increasing filtration. I’m not sure what your nutrient input is (whether you fertilize or potentially over feed) but that’s something to consider, usually the suggestion is whatever your tankmates will finish in like a minute or two and do a few feedings a day (unless you have just bottom feeders/shrimp then they wouldn’t finish food that quickly). And light wise, maybe reduce to 4 hours a day until it begins to clear up.

Bright side, because its cyanobacteria, it can also be taken care of with antibiotics like erythromycin, which should be safe for most tankmates, plants, and beneficial bacteria.

As mentioned, it probably is excess of nutrients caused by too much food or/and in adequate filtration.

That said, a water test would be the best what of answering your question.

Do you keep an eye on your parameters? Can you let us know what your ammonia nitrite and nitrate values are?

Less food, more filtration, increase surface agitation.

There’s some “blue green slime stain remover” you can use, as directed, which will help.

Suck up as much as you can, clean your filters and reduce feeding and light. That’s how you make it go away and keep it away.

It's down in your substrate which is a pain to deal with. Once it gets this bad no amount of water change and cleaning will fix it. You're going to need either something that eats the algae faster than it grows or chemicals.

One thing you can try is removing your substrate and treating it heavily in another container like a bucket. That'll be easier on any critters you have in your aquarium. You'll probably still need to apply some algae treatment in the aquarium as well. You'll want to clean heaters, bubblers, filters, etc. Change out all of your filter media as well.

If you have another tank you can also move any critters to that tank temporarily and do heavy treatment in the aquarium. Then do the cleaning followed by a large water change to dilute the algae treatment.

Best of luck, but be prepared for a fight!

Hmm, how new is the substrate? Is that the fertilizer substrate that comes treated? If it is, it might be leeching off way too much for the tank. You might want to add or swap in some sand if it continues to be a problem going forward.