I have a question on my green star polyps?

[Silk]

My GSP have had a brown coat of algae on them at various times before making them look brown. It could be that a snail has not cleaned them off. Other than that you may have a contamination of some sort that is killing them off. Not sure what it could be but those are my idears.

 

[smokin reefer]

Well they are now brown. I would like an explanation as to why they turned brown. Or any links to read up on this would also be help. They have a lot of water movement, plenty of light. Here are some of the tank paremiters:

Calcium-440ppm.
Alk-3.5meql.
ph-8.3, and steady
The last time I checked I had no nitrates,phosphorates,or silica.

I also use IO salt mix, add the kent turbo calcium, and supperbuffer. Along with the ESV mag. Any ideas on this would be appreciated.