What are those brown flakes in my reservoir?

[MrNorCal]

This is my second time doing hydro. I Just got done cleaning my reservoir and no matter how hard I try, I can't get all those brown flakes out of my tank. As soon as I turn the pumps back on they shoot them everywhere. Anyways, this happened last time and everything turned out fine. But this time around I just want everything to be perfect. So is that stuff old crusty nutrients? Algae? Is it bad to have it in the nutrients? How do I get it all out? Sorry for so many questions. Thanks in advance for any help.