Jack Magic's The Pink Stuff has HEDP. The
MSDS says it is a proprietary formulation and they don't say how much of that is HEDP, but if we assume it's 60% then with it's molecular weight of 206.02 g/mole and having two phosphate groups per molecule and with phosphate having a molecular weight of 94.9714 g/mole the initial dose of 1 quart per 10,000 gallons would be 1,000,000*((1/4)/10,000)*2*94.9714/206.02 = 23 ppm (23,000 ppb), but more likely it's in the 1-10 ppm range with maintenance. So if all the HEDP were to break down, then it would add a lot to phosphate levels. I know from personal experience that it can't be breaking down very fast or so completely, but my pool got to 3000 ppb phosphates after a combination of 1-1/2 years of using a metal sequestrant regularly (in a preventative maintenance dose) along with 400 ppb phosphates in my fill water (but I have a pool cover on most of the time so minimal evaporation/refill).
However, the phosphate level is irrelevant if the chlorine level is properly maintained relative to the CYA level. There are those on this forum with even higher phosphate levels of 5000 ppb. Now it is true that a pool is very reactive at high phosphate levels -- IF the chlorine gets low then algae grows more quickly but it still takes algae 3-8 hours to double in population under ideal conditions (i.e. with as many nitrates and phosphates as they need for optimum growth) because algae is ultimately limited in growth rates based on temperature and sunlight.
If you are using test strips for your 30-50 ppm CYA estimate, then that can be completely bogus. You have to use the turbidimetric test for any sort of accuracy. Also, 3 ppm FC with 50 ppm CYA is below the minimum FC of 7.5% of the CYA level and it likely fluctuates lower at which point algae growth increases chlorine demand and things fall apart fairly quickly. For saltwater chlorine generator (SWCG) pools, the FC needs to be a minimum of 5% of the CYA level and usually the CYA needs to be 60-80 ppm or else the SWCG is on too long which causes other issues of faster pH rise and wearing out the cell faster. So figure 4 ppm FC with 80 ppm CYA for SWCG pools and 3.8 ppm FC (so say 4 ppm) with 50 ppm CYA for non-SWCG pools.
We've seen this pattern over and over again in thousands of pools from new members to this and other forum (35,000 members on this forum, tens of thousands on others as well). With non-SWCG pools that you aren't visiting more than once a week, having the FC be at a good level one day doesn't mean anything because the next day or two it is guaranteed to be too low unless you are dosing with a peristaltic pump or The Liquidator. If you are dosing using Trichlor tabs/pucks, then they build up CYA. For every 10 ppm FC, they also increase CYA by 6 ppm so even with a 2 ppm FC per day chlorine usage that's an increase in CYA of 36 ppm per month if there is no water dilution.
As a pool service, if you want to use a phosphate remover or a weekly algaecide product then those are options for you (at extra cost, of course), but just realize that you can have the pools be algae free using chlorine alone but will take either automated dosing or a large range in FC swings (see the Pool Professionals thread I linked to earlier). The pool service in my area servicing 2000 pools uses Trichlor pucks/tabs, but with a 4.5 ppm FC target and they do a partial drain/refill of the pools to keep the CYA below 100 ppm. Even so, they still have some pools get algae, which of course is not surprising since their FC target only works up to around 60 ppm CYA, so they shock with chlorinating liquid and if that doesn't prevent a recurrence, then they use a phosphate remover. They know, however, that they can't just let the CYA keep climbing or else even algaecides and phosphate removers won't be enough. I also know this from my own experience in my first years with my pool where 3 ppm FC and an eventual 150 ppm CYA couldn't stop algae from growing even with Polyquat 60 being used, but only every other week.