Detritus/brown algae explosion, help

[Rodney]

Alright so I haven't been at this for very long so I figured I'd hit up the experts for help
The tank is 125 gallons roughly 180 lbs of live rock 3 anemone 2 pulsating Xenia, a very large colt coral , mushroom, 4 rock anemone and a few new LPS that I just added
Fish are a clown sweetlips, false perc, black oscilaris, coral beauty, manderin goby, couple starfish, hawaiian lobster, watchman goby and a regal tang and than around 3-4 dozen hermits and snails
Sump is a 90 gallon custom running filter socks and a psk300 with a uv sterilizer
Recently just upgraded the lighting to 3 250watt mh with 8 actnic t5
The water is testing good only thing showing is about 5ppm nitrate but my issue is what I'm assuming is detritus forming on the substrate and a brown algae starting to cover my rocks, seeing a huge explosion since I changed my lights, any advice or recommendations would be appreciated
Right now the only thing I can think to do is manually clean it when I do my water change and putting some mangrove plants in my sump to eat up extra nutrients
Thanks