It's also possible that the lights over the frag tank could be more "algae friendly". Lots of folks will put a much more white-light color over a frag tank since it's assumed that a 10,000K light puts out more PAR in general than a more blue 15,000 or 20,000K light (though that's certainly not a guarantee!).
I've found that the more blue the light is, the more "coral-friendly" the light is and less "algae friendly" it is. Most of our nuisance algaes seem to greatly prefer lower K rated light.