Biological filtration will help keep algae down as well, you dont have to just use carbon. Use some kind of ceramic rings if you can. If I'm not mistaken, the FX5 has a huge amount of mechanical filtration (foam) that pulls debris out of the water. Make sure you keep that clean, then have the other compartments full of some type of media like ceramic rings to grow good bacterial colonies on and you shouldnt have much trouble.
I'm not saying dont run any carbon, but it would be better to get your tank balanced out biologically so you dont have to keep replacing carbon every month.
Maybe try redirecting the output of your FX5 towards a rock that seems to have the fuzz on it, and in a week or two look at it and see if the algae seems diminished or if it's still going strong. That at least will tell you if more flow will help the problem or not.