I did some reading on the Barr Report (great site) last night, and Tom Barr seems to think it's more a lack of nitrate than excess phosphate. That still fits in with my experience as I was dosing less of my macro mix to get the phosphate down, so had less nitrate.
"Low NO3 is part of it, and often the main issue in newer tanks and tanks that are really clean and prepped well.
For other tanks, often clogged filters, a lot of organic matter, dirty filters etc, seems to be more the issue. Also, if you have a lull in plant growth in general, something it will infest some plants, a bit, maybe at the growing tips.
Excel might help a little, not much, but the overall effect of a large water change+ cleaning filters, + dosing more KNO3, + tweaking CO2 a bit more, etc, jujst run down all the basic items that you need to do and check them off and make sure they are in good shape."