Zoa Matt turning brown

[iMob]

I have quite a few high end zoa frags and the Matt and disk on some started to turn brown.
I really don't know if it's diatoms or what but I don't have any signs of algae in tank.
Should I worry?

 

[A. Grandis]

Yep, possibly diatoms.
Check your nitrates, phosphates and, most important, be careful not to introduce more silicates in the system.
Not to worry if it's under control. It's going away when you take care of the water you put into the system.

Grandis.