Phosphates are not the cause of algae growth. However, adding them will sure feed algae. These are two distinctively different things.
Algae are mostly caused by lack of nutrients in relation to light levels. To grow, plants need nutrients (incl CO2). In low tech (low light) tanks these might be supplied by natural processes, water changes, fish food and excrements, etc.
With higher light the nutrient requirements grow, so you need to dose. If you just dose one thing (say Flourish) your plants will miss essential nutrients, and stop to grow after a while. Algae takes over. If you have a bunch of algae for whatever reason and you add more phosphates, algae will say "thank you" and grow twice as fast. Trick is to ignore algae and provide your plants with all nutrients (did I mention CO2?) so that they will take off. After a while (this can take months) algae will disappear and everyone will be happy. Something like that.