That is typical with algae that these # are low or even undetectable.
I had always good luck with 3 days blackout but you can't do it to often as corals get stressed.
Lately what I have been doing and many think I'm crazy

lol yeah even I think so by times but I was surprised that my theory worked in the field too.
I started to think when the problem arises that the algae is taking the nutrients first before bacteria you need more bacteria with other words the balance of the eco system is off.
I corrected that by dosing Oyster feast with a bacteria mix AF and ZEO.
After a few days I noticed no algae film on the glass and a bit over a week the algae was turning grey.
I tested daily on Po4 and No3 to see if # came up and they did indeed but slow.
When I noticed this was working I knew the bacteria were kicking in and fed a bit more on the Oysterfeast.
After two weeks most algae was gone and my Po4 sat at 0.04 and N03 at 5ppm that's were I like to keep it on my DT and never have a algae problem in there.
This test was done on a frag tank 75 gal 60"x24"x10"
Conclusion was that I was starving my bacteria population and my red field ratio was way off.
NOTE: I had to dose ALK and K a lot as these # were dropping as well.
Not saying that this might work for you but dosing some extra bacteria when you feed the fish to see if it has some impact.
Reducing your white light photoperiod from 7 to 3 is not bad thing to do.
White light/daylight is the main light source for algae to grow on.