How old is your system? What animals do you have to eat the algae? Various algae cycles, colloquially called "the uglies", are normal during the maturing process and often happen after tank moves and if there have been disruptions to the ecosystem like power outages or equipment failure. I only use manual removal to deal with nuisance algae in my maintenance business and over the decades have not seen any correlation between PO4 and nuisance type algae. If you have corals I would not be dropping phosphates but adding phosphate to have some measurable PO4, prefferably above .03 mg/l.. Corals need phosphorus and are competing with algae for it. Stripping out phosphate can have very deliterious effects on corals potentially killing them as well as making it easier for algae to compete. Here's quotes by J. E. N. Verone and Charles Delbeek, two of the formost authorities on corals and some links if you're interested in reading further:
"Our crystal-clear aquaria do not come close to the nutrient loads that swirl around natural reefs. And so when we create low-nutrient water conditions, we still have to deal with the rest of a much more complex puzzle. Much like those who run their aquarium water temperature close to the thermal maximums of corals walk a narrow tight rope, I can't help but think that low-nutrient aquariums may be headed down a similar path." Charles Delbeck, Coral Nov/Dec 2010, pg 127
"Imported nutrients are usually transported to reefs from rivers; but if there are no rivers, as with reefs remote from land masses, nutrients can only come from surface ocean circulation. Often this supply is poor, and thus the vast ocean expanses have been refered to as "nutrient deserts". The Indo-Pacific has many huge atolls in these supposed deserts which testify to the resilience of reefs, but the corals themselves may lack the lush appearance of those of more fertile waters. Many reefs have another major supply of inorganic nutrients as, under certain conditions, surface currents moving against a reef face may cause deep ocean water to be drawn to the surface. This "upwelled" water is often rich in phosphorus (.2 mg/l) and other essential chemicals." J. E. N. Veron "Corals of Austrailia and the Indo-Pacific" pg 30
https://therichross.com/skeptical-re...and-phosphate/
https://www.cambridge.org/core/journ...D254623FD3C7C#
An Experimental Mesocosm for Longterm Studies of Reef Corals
Phosphate Deficiency:
Nutrient enrichment can increase the susceptibility of reef corals to bleaching:
https://www.nature.com/articles/nclimate1661
Ultrastructural Biomarkers in Symbiotic Algae Reflect the Availability of Dissolved Inorganic Nutrients and Particulate Food to the Reef Coral Holobiont:
https://www.frontiersin.org/articles...015.00103/full
Phosphate deficiency promotes coral bleaching and is reflected by the ultrastructure of symbiotic dinoflagellates
https://www.sciencedirect.com/scienc...601?via%3Dihub
Effects of phosphate on growth and skeletal density in the scleractinian coral Acropora muricata: A controlled experimental approach
https://www.sciencedirect.com/scienc...22098111004588
High phosphate uptake requirements of the scleractinian coral Stylophora pistillata
http://jeb.biologists.org/content/214/16/2749.full
Phosphorus metabolism of reef organisms with algal symbionts
https://esajournals.onlinelibrary.wi...Vm0sG8_0vth6lq