Green film algae confusion

[WhoPoopWrasse]

I'm a little confused!
Two years ago when I was new to all this saltwater fish tank stuff I was having a very hard time keeping my nitrates (20-40) and phosphates (0.25-1) down. Mostly caused by over feeding, overcrowding and not enough water changes bla bla bla. At the time I would get green algae on the glass which I had to clean daily so I asked questions. (wasn't on this site)

Answers were:

More water changes - so I started doing 15% every 2 weeks

Cut down feeding - Now I feed 1/2 of what I used to and fish are still plump

Too many fish - I removed 2 big fish (tangs), I still have one

Add macro algae to the sump - I did and readings of both nitrates and phosphates were at 0

So here is where I'm confused,

Before I started Zeo and my readings were 0, I was still getting algae on the glass because what I was told was, even though readings are 0, I still had lots of nutrients in the water and the readings said 0 because the macro algae was sucking it all up.

I have since then started zeo, (June 3rd) removed all the macro from the sump (over 6 weeks ago) so now nutrient export is via w/c which is still 15% every 2 weeks.
My reading are now phosphate 0 and nitrates between 2-5.

My question after all that explanation is:

Why is there still green film algae on the glass?