New to reefing

I've been into aquarium's for 15 years now. I'm only 9 months into my first reef tank. Everything been going great except for the last month or so. Some of my corals are not opening up like they use to, also starting to get a lot more green hair algae . I have a 28 gallon nano cube intermediate LED with an aquamaxx hob protein skimmer. I do bi-weekly 5 gallon water changes. Nitrates are 5 ppm. Need some tips on what I can do different.I have a hammer coral, colt leather, couple zoanthids, 2 Duncan corals, and a acan. I run the main lights for 6 hrs and the moonlights 1 hour before and after.