Looks like red cyanobacteria to me, which is relatively common. If you can use your hand to create come local current, and it blows off, I am almost certain it is red cyano. There are a few ways to deal with it. In short you have too many nutrients, so water changes will help, but there are more efficient ways. Manually syphoning as much as you can out is the best thing, then replace with new salt water.
In my tank, I like to use Chemiclean for how efficiently it worked for me. I do modify their recipe and only use ~1/4 to 1/3 a dose, and I drain the wet skim out of the tank and replace with new salt water (I am actually doing it as I type this) as Chemiclean causes the skimmer to froth up and overflow. Pictures of what I do are in my tank journal. Let me/us know if you have any more questions!