It's probably not a well accepted theory, but cyano bacteria has a disproportionately high demand for iron, and they've evolved family specific pathways to harvest it from the environment called siderophores that might be able to pick up the form of iron found in GFO. Research around shipwrecks and black reefs have recently shown a strong correlation between the grounding/sinking of iron based ships, and the rapid decline of surrounding reefs as simple algaes like cyano over-take corals. A recent paper linked it directly to the iron rusting out the hull.
There are also half cocked plans to dump enormous amounts of iron in the open ocean to encourage phytoplankton as a form of carbon sequestration. A great deal of the phytoplankton in the ocean is free living Cyanobacteria. Different genera than the kind on your sand bed, but the kinds we deal with have many of the same needs and capabilities.
Anyway it's clearly more complex, and there's more to cyano outbreaks than just iron, but I have anecdotally experienced drastic changes in its behavior just by starting or stopping the use of GFO. I use PO4x4 now, which isn't granulated, it's polymer encapsulated. Produces zero dust and theoretically releases less iron in to the water column. It's expensive as all get out, but I haven't had any issues with cyano since.
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