Bad Water Quality?

[hummer]

Hi Everyone
For the last 3 weeks the stuff collected in my filter socks and skimmer have been black. Different from it's usual brownish color. Is there something wrong with my water quality?

Everything in the tank seem healthy and the water is very clear. I tested Alk, Mag and Ca and they are fine. Salinity is at 1.025. I only use RO water.
Around the same time I notice the black stuff I had put in fresh carbon (Black Diamond). It's the same stuff I've been using all the time and its never turned anything black. And I rinse the carbon before i add it. I thought it might be the carbon but after 3 weeks it should have cleared.

Any ideas?
Thanks!