What is growing on my zoas?

[olavvk]

Hey! I have these zoas now for about a month. Normally most of them open up normally but i have some problems. So as you can see in the picture, it seems like something is growing at it. Last week I dip them in coral dip again and cleaned them with a toothbrush. It was gone, but a few days later it came back. Does anyone know what this is? And how to solve?

 

[shakacuz]

judging by your sand, i would say diatoms and detritus buildup. i would increase flow all throughout your tank to get the dead spots moving.