SAM,
Kent's phosphate sponge will work a little bit. Follow the directions, I think it should only be in your system a few days at most.
But you are only fixing the symptom and not the problem.
You need to identify why your phosphates are high in the first place. (if they are) You don't mention test results.... just the cyano..
Phosphates can come from many sources... Your water source, over feeding, chemical additives.... You need to identify the source to make a real difference.
As far as dealing with the
red slime. You can start by removing it with a syphon, and getting some more circulation in that area.