Green spot or dot algae is actually not a true algae; it is a population of diatoms, microscopic animals that secrete a hard silica shell on which green alga grows and that is what we see. [So-called brown algae is also diatoms, thus not a true algae either.]
Here is some good info with photos from a Swedish aquarist, Dusko Bojic, just to explain it. I would not fuss over the suggested fixes though, in my view this is irrelevant. I will see this sometimes, a couple spots, if I forget to clean the inside glass on the front every water change. The trick is getting it before you can even see it, and just going over the glass with a sponge scraper during the water change will do that.
Green Spot algae
The 3rd photo is made with a macro lens. Those green dots are smaller than 1/2mm.
Green Spot Algae prefer direct light. It forms green spots on aquarium glass and slow growing plants that are exposed to strong light. This algae will appear if CO2 and Phosphate (PO4) levels are low. Since it is very hard, algae eaters can't do much in eliminating this algae. Neritina Zebra snail is the only algae eater known that can, literally, eradicate the Green Spot Algae. It can be scraped manually off of the glass with a razor blade. In a case of an acrylic aquarium use plastic razors only. This algae is considered normal in small amounts.
To prevent this algae keep stable CO2 levels, dose NPK regularly and improve the water circulation throughout the tank for better nutrient transport. Keep slow growing plants in places where they will get less light. [written by Dusko Bojic]