I've never seen that, maybe someone more experienced will chip in and say what it is and offer a better solution, but I am always a fan of doing a 3-day blackout once a month for three months. Blues on the fourth day with a water change to try and manually suck up any algae that is weakened.
Nothing in this hobby happens fast, so it may take a few months but if you keep up with water changes you'll get the algae under control.
But like I said, I don't know what kind of algae that is, if it is algae, so maybe someone else can lend more information.