Sounds like the yellow pigment of dino's.....maybe chrysophytes, but that's less likely.
You're creating the optimal environment for whatever it is by holding phosphates down at zero.....unnatural and unhealthy for almost everything in the tank.
BTW, you also have a ton of livestock in your tank that LOVES high phosphate levels.....in particular the anemone, but
literally everything else too.
Adding the algae reactor to your GFO usage did not help matters....macro algae also requires phosphates.
So you're trying for simple, but you need
even simpler.
Balancing your nutrients needs to be your new goal.....artificially removing nutrients needs to be a thing of the past for this tank.
Once you balance the N in your tank with a little bit of dosed P, you'll start to see improvement.
If the dino's (yellow growth on glass) should
automatically go away after a day or two of P-dosing, though it's possible you will have to be a little more persistent.
Not only will the dino's disappear, but the whole tank should be a lot happier. You'll most likely experience a coral growth spurt as well....so keep an eye on NO3 and PO4.....
don't let either one get down to zero.
2¢
Let me know if you have any questions as I know this is a lot to take in and that it may be contrary to a lot of what you've heard in the hobby so far.....you're welcome to PM or ask here if you have any questions at all.