In the first year anything is possible. As
@db75Reef said, PO4 can be 0 on a test, but this only indicates dissolved in water, not part of the algae. Scrub the algae, increase flow and remove algae manually.
The first year, things will be wonky. Was your rock live rock, dry rock or cured and cultured prior to introducing to your system?
In my case, I always start with dry base rock & it is not unusual to have alage 6-8 months in FOR ME. I don't get to crazy for the first year. Back in December I moved a tank 50 miles, and the disruption period is almost over. Things are starting to stabilize. I know this from observation, and don't test for anything other then specific gravity. It is a catch 22 really. You need fish to produce waste which feeds the bacteria during this time, but also the tank can and may be very "fragile" on what changes it can handle on its own. There is no one-size-fits-all solution. Each tank is different. I have had great luck with vinegar dousing, but it is my method, and am religious about it.
Water changes are clearly not happening often enough.
Once a month/ two months at best.
I need to scrub it down and change the water 50%, see if that helps.
The best thing to do to verify is become regular with water changes. 25% every other week is good GENERALLY. Larger/more frequent changes may be required to get algae under control.
I know that if I go a month without a change AND there are any other issues, it will produce algae. Once you get algae, you have trapped algae food. You can become perpetually algafied (not a word, but I made it up anyway). Algae dies, and releases the nutrients back into the water, and more algae just eats it up. This is where heavy water changes and physical removal of algae works best. You are removing it all.
It can take months for a wrong mistake to be worked out, and it requires work.