New and need help

[City Reefer]

Hello...
I'm new here. Been into fishkeeping for years but I'm still learning about reefing. I've had my tank up and running for about 3 years now and recently started adding corals.

I love them but other can be a pain in the butt!

I have a 110gal display with about 30 gal of sump. I have a large amount of live rock.
I have a SCA 302 protein skimmer, fluval sea led lights and orbit led lights. I have gyre xf150 and jeabo cp40 plus two jecod rw8's.
I have
1 yellow tang (small)
1 Nason tang (small)
1 maroon clown (large for clown)
1 snowflake clown
1 saddleback clownfish
2 3 stripe damsels
3 chromis
1 dottiback

1 carpet
1 bta
Various corals (please see avitar)


I have always had a nitrate problem with nitrates hovering around 40ppm. No phosphates. Cal at 500.
All other parameters are great.
Never had an algae problem......till now.

I recently added biopellet reactor and a reactor for seachem de☆nitrate. I've also started dosing hydrogen peroxide.

After 3 days of having the de☆nitrate reactor up and running, I'm starting to see green hair algae for the first time in this tanks life.

I don't know what's causing it.
I'm asking you guys to help me out of you can.