It is probably blue green algae, though a pic would confirm it, a three day total blackout would kill it. A 50% water change and a filter clean removing as much as possible, then a 50% water change after the blackout to remove any dead algae. Antibiotics also work, though I have no experience of using them. Low nitrates are supposed to be a trigger for it. Egeria densa is a very easy plant and is said to be able to supress BGA, though I have had lots of it and BGA.