Did you see my questions about additives or things you put in the tank?
From the picture, it's possible it's an algae bloom, but it's also possible that the green hue from film algae is "coloring" a bacterial bloom. Either way, here's what I would do:
1) If you have access to a UV sterilizer, install it in the tank. Should clear up quite quickly.
2) If not, I would perform some large water changes. Make sure to match salinity and temperature so as not to harm your fish or other organisms.
3) Discontinue dosing any carbon source or excessive food
4) Increase aeration in the tank- use a powerhead and point at the surface of the water and/or install an air pump/air stone to increase oxygen to the water.
5) Measure your phosphates/nitrates. They may be artificially low in the case of an algae bloom, but it's useful to get a baseline.
6) When you are performing those large water changes, if you have that green algae all over your glass, scrape it off and suction it with the hose you're using for your water change.
7) Double check that nothing in the tank has died. If so, add some Prime and test your ammonia. If everyone is fine, skip this step.
If you can tell us more about the tank, we can try and figure out why this happened.