Going with Polyplaps is likely not going to have a profound effect on your tank. It is primarily a nitrate reducer. Since your tank is at 1ppm we pretty much can rule out nitrate as a cause of trouble here.
I don't think we can rule out PO4 though. I'm not sure what to think of the 0.04 reading. I get this kind of reading too on my tanks but the lack of a reference solution gives me this whole "how can I verify this number is correct" thing going on. I've been on the fence about ordering a Merck/D-D test kit but as time goes on and my PO4 numbers all over the map, I'm starting to think I'm going to have to bite the bullet and just order it.
It might not be the PO4 though either. Well, in so much that it's not the PO4 in the water column. There may be PO4 trapped in the rocks though and it's slowly leeching out, fueling the GHA.
The lack of coraline on the one rock to the front left there is something that catches my notice everytime I see a photo of your tank. I realize it has some, but ... I dunno, the rock looks white to me. When I set up my 20g back in December, I pulled rock out of my rubbermaid curing tub. It's been sitting there for months and was devoid of any coraline, was more or less pure white. I did get some minor hair algae growth after a few weeks but after about a month it was mostly pinks and purples due to coraline coverage, and where the coraline takes hold, the GHA doesn't. So it's odd to me that this one rock isn't more covered in reds and pinks and so on.
I think, were this my tank, I'd be looking at swapping out most, if not all of the rock. The existing rock should be thrown into a bucket for cooking (just let it sit in some SW with a heater and change out 100% of the water from time to time). And new rock put into the tank. Maybe not all at once though.
Any idea how many lbs of rock are in there now? You might want to consider upping the amount that's in there too while you're at it (couldn't hurt - except for maybe the cost of course!)
Hard to say if those Ca/Alk #'s tell us the reason for slow growth. They are both lower than what you might want to shoot for, but not hugely so.
Try upping the Ca to around 390-400 and the Alk to around 7-8 dKH and see if you notice a difference.