It is under control at the moment. I gave it a good clean and a 72hr blackout which has knocked it back completely. We will see if I get a recurrence. Nitrates and phosphates are low and under control.
Is it only on eggcrate? Some eggcrate leaches phosphates and encourages algae growths... Looks like new eggcrate so that made mention about this, sorry can't help with ID.