Are you using something like easy carbo? Than likely is dying BBA and will go away in a couple of days or weeks.. Or it is kind of beard algae (Rodophyta - red algae) in most cases it looks black and turns red when dead. But also can look red in circumstances or sp. depended when alive. Most realy red algae sp are found in marine invironment, in fresh water this is rare, but not impossible, i've only experienced it once, having living red algae growing in my tank.
It can be treated with easycarbo or H2O2 to kill it.