To me, it looks like a macroalgae, but I'm bad at id's. If it breaks up very easily under your fingers, like a layer of mud, almost, it's a microbe of some sort. In most cases, whether it's a macroalga or a microbe, nutrient control will work to control it. Less feeding and a GFO reactor are two of the easier approaches. Better skimming might help, too.
Could be Bryopsis. I think it's definitely an algae.