My reef has been running 6 years and the glass "algae" has changed dramatically as the tank evolved.
First year or two I had high nutrients. GHA grew, cyano & Dino's came & went, zoas, mushrooms, ricordias thrived and grew. True green algae on glass needed removed every day. Little to no coralline.
About year 2, everything changed. It was the low nutrient phase. I had to knock nutrients down to defeat bryopsis. Employed detritus removal, mechanical filtration, bio pellets for a while & most importantly GFO which I've run continuously. My hard corals were growing but needed to dose Alk/Ca/Mg so set up the auto dosers. Coralline really grew on the glass to the point of becoming a PITA. The glass "algae" turned from a green color to a brown film. To my delight the brown diatoms weren't noticeable for up to 2~3 days at a time. Pest algae on LR was light & manageable. It might show a little if I slacked on changing GFO but wasn't a problem. During this time I believed the green algae essentially bailed out of living on the glass and was out competed by brown diatoms on the glass perhaps. My filter pads were efficient capturing & exporting the diatoms. The tank was on cruise control.
About a year ago I decided to increase target feeding my LPS & feeding planktonic food to SPS corals and some others. I backed off GFO a little and limited my wet skimming. Coral health improved and colors got much richer & vibrant. Algae like GHA & a small patch of turf & some Valonia (bubble) increased just a tad, but was manageable. But I have to be careful & vigilant not to let the pendulum swing too far in the other direction.
In this new environment I was surprised to see the brown diatoms on the glass reduced even further. Now 4~5 days can go by without the need for a glass cleaning. You'd think the opposite would be the case as you increased nutrients. Maybe the corals and increase in other algaes began to out-compete the diatoms on the glass? Coralline grows the same as always at 7.5-8 Alk 420 Ca 1300 mg.
Like the poster above said, what I now have on the glass can best be described as a brown colored bio film. It's probably a combination of diatoms and a bacterial film. It appears to no longer contain any green algae. It's really interesting to observe these subtle changes that happen over the years. It's a reminder that our aquariums are always evolving somehow.