Dear bro,
According to Dr. Robert J. Goldstein in <Marine Reef Aquarium Handbook>
"For most higher plants and algae, nitrogen is the limiting nutrient: when it is used up, the plants stop growing. Blue-greens (Cyanobacteria) are not nitrogen-limited because they typically fix atmospheric nitrogen gas (or in aquariums, dissolved nitrogen gas) into all nitrate they need. They are however, limited by the availability of phosphorus..... Only phosphate starvation is safe in a reef tank."
With your reading of low phosphate, I believe your problem lies with orthophosphate which is not reflected in common test kits.
Slicates have been link to diatoms bloom rather than cyanobacteria, also silica alone will not cause a bloom unless there are abundance of nitrates as well.
Beside siphoning and frequent partial water changes, you may want to try the below:
1) Use distilled, de-ionized or RO water for water top up and salt-mixing
2) Feed sparingly and ensure the food you feed does not contain too high phosphate
3) Use Phosphate removers such as Rowa-phos etc
HTH