the algae eats your phosphates faster than it is made, so it will register at "0", even though you have them. that is the food for algae, It comes from excess nutrients in your water. Try reducing feeding for a time and reducing the lighting cycle by half for a period of a week. Likely, the problem will subside. If you are totally desperate, there is a product called chemi-clean, which is reef safe but only has a minimal effectiveness (meaning it may only work the first time you use it as the algae develops an immunity to it) It will not solve the problem of excess nutrient, but will get rid of the slime while you cut back on nutrient in your tank.
-Doc