The Lack of Co2 & the fact your lights were still running did'nt help, I'm sure.
To start with manually remove as much of it as possible. Do a water change or two, get your Co2 cranking & test your water, Nitrates etc. see what your lacking or low on, dose accordingly & give it a few days & see what happens.
Spot treating with flourish or tank blackouts maybe an option after that.
Algae Architect....Yes! I have Algae, but it's all very tasteful...