What is this? Please chime in :)

[QuarantinedCorals]

Hi guys, this is a friends tank I'm trying to help him figure out what this stuff is. Any ideas on what this is?? His phosphates read at 0.01 and nitrates at 0.

 

[kzenoni]

I get that in my tank, changing the carbon in my sump usually clears it up in a day .