Main issues generally too much light, not enough water changes and too much nutrients, like excess fish food waste.
That looks like too much light possibly causing some issues. Are you doing frequent water changes? Feeding a lot of flake food? Someone told me to think of the size of the fishes eye and that's about how much he needs to eat. Baby fish more often. Some flake foods I think have more phosphorus than others which also encourages algae.
How long is the light on for on the tank? Does it possibly get sunshine coming through a window?
You can try cleaning it out and do a 3 day black out. Or longer if there aren't any live plants/ or live plants worth saving left.
Just get your hands and arms clean and well rinsed, and go to town pulling out the things from the tank, pick it off as much as reasonable and spray with H2O2 Hydrogen peroxide 3% like at the first aid section of the store. Walgreens has a spray bottle with peroxide, can't recall where else I have seen it. I have some algae issues and am planning on trying these steps.
Let it sit for 5-10 min while you do the next item, and then rinse the one and you can try an old well rinsed out toothbrush to scrub it too.
Or if that doesn't work try the same thing using Seachem Flourish Excel. I am not a huge fan of using this type of product in the tank, like Glutaraldehyde because it could become toxic to invertebrates. But treating the things in the aquarium and then adding them back seems okay. I am not too sure how long to let it sit on rocks and decorations.
For plants, you wouldn't want to let them dry out, so maybe a wet paper towel over them.
Doing a good water change the next day would be smart, as they algae will be dying off.
Some have said they dose H2O2 into the tank water, learned of it in reading but haven't talked with some one who does it.
edit - I always used to use Phosguard or Purigen, and after I moved last year, recently I realized I hadn't put those back into the filters. Maybe a correlation, which those products assisted if preventing the algae grip.
