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!