Your flow is fine.
It takes 22,000-30,000 µW/cm2 to kill algae...
your FX5's flow is around 500gph...
So your running that UV at a rate of around 45,000 µW/cm, which is more than enough to kill algae...
So you UV and flow rate are perfect for killing algae,
If you slow the rate, the UV will have a higher µW/cm2 rating, which means it will kill other things in the tank... but doesnt make it any more potent to algae...
The thing you have wrong here is your wattage... that UV is too small for that tank.
So either get a bigger UV, or stop feeding the algae with excess lighting periods, or excess food. Also, do more water changes to lower your nitrates. If you do that, that UV size will keep algae in check.