Algae or something else?

[chadg]

So, on my skimmer in my return sump chamber, I'm getting this growth in the past month or so. The tank is about 5 years old. Before this the skimmer pump stayed pretty white for me.

PO4 is .15, Nitrates 5-10 usually. Alk 8.1. It looks more like a fungus/mold vs. algae. It starts out purple and is now turning more black. I have cleaned if off a few times, but it comes back. Any ideas?

 

[Webslinger]

algae ,sheets of black plastic are your friend. they block light

 

[chadg]

It is my sump, but not under a cabinet. It is out in the open in my basement. Good idea on the black plastic. I'll block out the entire sump and see if it goes away. Thanks!