What's the grain size in your sand bed? According to
this experiment fine sand, 0.2mm and less, is said to reduce phosphate somewhat. Maybe something to add to the fuge? I wouldn't worry about the cyano too much if it is only on the sand, its just cleaning the water for you and will go away on its own. I had a terrible mess of the stuff in my tank for about a month, but it is mostly gone now with no chemicals needed. It was so thick I was able to pull sheets of it out - a good nutrient export. But, seeing as you have a bunch of corals in there that risk smothering you might not have that option if it starts working its way up the rocks.