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 with a 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