cyanobacteria(red slime) eats nitrates like crazy, you will prob test real low for nitrates cuz of the cyano, more flow might spread it to your display, I would physically remove it, however, your nitrates may go up in which case you will need nitrate control. 20% waterchange would be quickest, macroalgae in sump, denitrator of some sort. also what is your photoperiod in your sump, you may have to reduce it.