Brown stuff on marinepure balls?

[Fishfishfishfish]

I have brown stuff on my marinepure balls in my sump. Is this build up of poop and what not, or bacteria colonies? I don't use filter socks because I hear they are bad for nitrates but would they prevent this? My tank is 5 months old.

 

[PDR]

Looks like detritus. Running socks changed out 2x a week would help with that.