White buildup on rocks?

[Theulli]

I am observing a white buildup on some of my rocks. It does not blow off with a blast of water or air - anyone know what this is? Alkalinity precipitation?

Params are within desired ranges. Corals are happy.

 

[Jekyl]

Can't tell. All I see is blue.

 

[Dburr1014]

Yup, can't see a thing.