I dont think its an algae but a type of bacteria not sure what eats it though usually I see it in takes with a decreased flow in the areas that it shows up at. on top of powerheads undersides and backsides of rocks, and on the back walls of the tank. Once it starts to grow it grows into the current areas from the lower current areas.
I always used a turkey baster to clean it off though but even then some remained and wouldnt BLOW away.
Could be an algae though not real sure but to me it grows and acts more like a bacteria than algae.
Mike