The most common cause of heater corrosion and deterioration is low alkalinity and low pH. You’re correct, phosphates are food for algae and shouldn’t be related to heater issues. On the other hand, there is a term called calcium phosphate scale, and this could also cause heater issues, if scale is forming inside the heater.

I did a quick search to see if in either definition there’s a mention of phosphate, or a similar term. I found:
Alkalinity is the capacity of water to neutralize acids. This capacity is caused by the water’s content of carbonate, bicarbonate, hydroxide and occasionally borate, silicate and phosphate. pH is an expression of the intensity of the basic or acid condition of a liquid.

A phosphate is chemical derivative of phosphoric acid. The phosphate ion, is an inorganic chemical, the conjugate base that can form many different salts. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid.

Nitrogen and phosphorus are nutrients that are natural parts of aquatic ecosystems. Nitrogen is also the most abundant element in the air we breathe. Nitrogen and phosphorus support the growth of algae and aquatic plants, which provide food and habitat for fish, shellfish and smaller organisms that live in water.

What about Phosphoric Acid?
What is phosphoric acid used for in water treatment?
On account of their special properties, phosphates and purified phosphoric acid help to reduce the risk of pipework corroding, prevent the deposit of limescale, control the pH of water, and control red and black liquors. Fluorine derivatives such as NaF and SSF are used in the fluoridation of water.

What does phosphoric acid clean?
Phosphoric Acid Cleaner is a fast act- ing, easy to use acid-base cleaner for pre-cast concrete, concrete, masonry, grout, tile and natural stone surfaces. Phosphoric Acid Cleaner will remove cement grout haze and mortar residue, joint cement, lime & hard water deposits, efflorescence, rust and other dirt and grime.

Phosphate Scale: Though calcium phosphate scale is a newly discovered problem for the swimming pool industry, it has long been recognized in the boiler industry, where the practice of using polyphosphonates to control calcium carbonate scale is often used.

Eventually the polyphosphonates break down to orthophosphate, which then bonds with calcium to form calcium phosphate scale. Calcium phosphate is found in phosphate rock and is largely mined for the production of certain cleaners and detergents.

Scale is formed in recreational water by precipitation and crystal growth on surfaces in contact with water. Three of the common mineral compounds in water are calcium carbonate, calcium phosphate and magnesium silicate. These three compounds remain soluble in lower-temperature water. In higher temperatures they supersaturate and precipitate on surfaces in the form of scale.

Calcium phosphate scale is formed in water when minerals containing calcium ions combine with orthophosphate. The result in pools and spas is an insoluble white scale on the surfaces of pools and equipment. Since calcium phosphate precipitates at higher temperatures, it tends to be more of a problem in heated pools or spas. Precipitated calcium phosphate can lead to the destruction of heat exchangers by coating the heating elements — preventing heat from being dispersed and potentially burning the elements out.

What causes it?
One of the possible reasons for the increased appearance of calcium phosphate scale in swimming pools could be the increased use of metal and scale remover products, which contain phosphates such as polyphosphonates (HEDP) or phosphoric and phosphonic acid. When these products are used, the phosphates eventually end up in the orthophosphate form.

In pools where orthophosphate levels climb and high calcium is present, calcium phosphate could precipitate in higher-temperature water. In the orthophosphate calcium scenario, tri-calcium phosphate is formed from the reaction of orthophosphate with calcium at the heat surface. SOURCE