Diotoms are the slight brown film that can develop on the sand, rock, and glass, not at all bad, many animals graze on it all day long.
Try increasing the oxygenation of the water, and cutting back to actinic only for a week or two.(if you can)
Also take the frag rack out if you can, algae will grow on plastic very quickly, (almost any marine algae and coral will), I moved mine to the sand, and while its higher maintenance, I lose less frags over all compared to the racks covered in bad algae that the grazers cannot get to.