Sam,wow - you have a lot of coral already huh!
I wouldn't be too fussed about the white algae, especially when the rest of the tank is looking fine. You didn't by chance add an urchin to the tank recently? They will eat coralline algae and leave behind a white trail like that
No, no Urchins. What really got my attention was the speed at which the new white/pik covers an entire rock that was already showing good "normal" Coralline growth.
My suspicion is the dosing I have been doing for Iodine ( Seachem)
Tested my Iodine again yesterday, ( Salefert) and still getting a bit confusing readings.
First part of the test is for the Iodide - and it had shot up 3 days ago to the yellow section, so around 0.1 whereas the target is 0.03~0.06
Then when I do the second part of the test for Iodate, I get a slight coloration only, which indicates 0.02 ppm
Until I get a clear understanding of this, I stopped any Iodine dosing 3 days ago, put my Rowa Carbon sock back in the sump, and will observe.
Incidently, as per water test recommendations, I removed the Rowa Phos reactor, have been feed well, and forthe first time in nearly 5 months, I got a reading for Nitrate of 0.5 and Phos of 0.03 :-)
I would appreciate any insights, suggestions or advise.