I suspect it'd be filled with GHA if the tangs weren't there. There is plenty of algae to be seen on the glass.
It's hard to say one way or another how it is doing. But for what it is, the current setup is about as good as it gets. It's a high-nurtient system. Great for growing soft coral, algae and pods. Considering the tank has soft corals, tangs and a dragonet...perfect. If it were set up the way a lot of people on this forum would like, it'd probably be a disaster.