Phosphate

[Myka]

Was your rock bought from a store (cured or uncured?) or from another reefer? It's quite possible the rock is leeching the phosphate. What brand test kit are you using?

Btw, you have it a bit backwards, hair algae is much more of a "dreaded" one than diatoms. Diatoms are not usually a very big issue at all, unless someone has confused dinoflagellates with diatoms.