Okay UG - Let's try to figure this out.
1. What is the texture of the algae (is it soft, slimy, rough?)
2. Does it have a strong smell, light smell, or not a noticeable one?
3. How old is your tank?
4. What is your lighting, schedule of said lighting, size of tank, fertilizer schedule, and co2 method (diy/pressurized)
It looks like a combination of GSA, GDA and diatoms. Green Spot algae is rough to the touch and is difficult to remove without some force.
Green Dust Algae is green to brown in color is relatively easily to remove and when first starting out, is rough to the touch.
Diatoms are from excess silicates, low lights and new tank setups. They are easy to remove and usually go away on there own.
If there is a strong smell and it is slimy to the touch, it is possible it is Blue Green Algae which is not an algae, but a bacteria. This is treated successfully with black-outs, nutrients balanced, or even erthomycin (a fish medicine).
Take out your co2 diffuser and clean it. The algae build up is interfering with the distribution of co2 and compounding the issue. You most likely have an imbalance of lighting, co2 or nutrients in the tank.
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