Coraline Algae Questions...

I do not understand why purple coraline algae doesnt grow on the glass in my tank. My rocks are covered purple/orange/red coraline majority of it being purple however I only ever have a little bit of red growing on my glass. Why would this be?



Calc - 430

Alk - 9

Mag - 1360

Ph - 8.3

Salinity - 1.026

Water Volume - 270 gallons