ident Algae how to treat?

[banditpowdercoat]

It's a 50GPD unit. membrane hasn't been changed in a while too.

I just did a 15g change. checked notrates and Phosphates of change water, both were 0. running sump into a double filter sock for now. ALOT of fillament flaoties in the water. I brushed off the rock, siphoned as much as I could but still plowing off rocks once tank settles. Also, when I siphined the sand, there was black and brown hairs in the sand? Like a shag carpet, just below the surface of the sand. I bet that's not good HUH?? Assuming it's poop? I dont have much for sand sifters, but didnt want a bare bottom. maybe i should rethink that??