We had this problem and found out it was coming from the store bought salt water we were buying from the LFS
We removed it with a couple of rounds of phosphate remover pads and water changes. That worked very well.
But Seaham is right. You have to figure out where it is coming from or it will just happen again. You mentioned in another post giving your tank a good cleaning a few weeks ago. When you cleaned your tank, did you wash anything in tap water? How about what are you feeding your tank? If it is frozen brine or mysis are you defrosting it in tap water? Even if you don't (and you defrost in tank water) I was told you should drain it because the water it is frozen in can contain Phosphates. Also are you running carbon? I heard that can contain phosphates sometimes.
I'm by no means versed on all this, but having just been through it myself, I can tell you, you have to become a detective.
What are you using to test the phosphates?