I was having some small algae issues (just a few strands of hair algae) and couldn't figure it out for the life of me. Everything tested good, with new Salifert kits (nitrate under 10, but detectable, phosphate 0.03 on Hanna ultra low, ca 440, mag 1430). I changed my filter sock, deep cleaned my skimmer and pumps, even pulled my plumbing and cleaned that. I was about to give up when I glanced at my lights (48" SB Reef led) that I've had, and loved, for a few years. The cover that protects the diodes from splash and salt creep was, well, doing it's job. It was covered in salt creep, so I tested PAR at the bottom, center and got under 150. I cleaned the light really well and retested to see a pretty amazing difference at 225. This was a couple weeks ago and I've seen the algae dropping off and my corals looking healthier and brighter since.
Just a reminder that when we get tied up in the numbers and the chemistry of the hobby, sometimes it's the little things we overlook.