It sure looks like blue green algae to me. And, is this a 180 gallon tank? If so, that is a lot of light for it. I suggest first decrease the time the lights are on to 6 hours, and wait a couple of weeks to see if that corrects the problem. If so, and you want the lights on longer, try just running 2 lights for 8-10 hours, with the third one on for only 2 hours or so in the middle of that time. Or, possibly better, raise the lights another 6 inches. BGA really digs light!