okay have done this myself!!!!!!!!!!!!!!!!
Looks to be a new system - so first off check all your parameters.
When Ammonia, Nitrite, and nitrate are all at zero get a cleaning crew appropriate
for your tank volume! Cut down on your light time. Make sure you are using Ro water - check it's parameters too! Once you have ordered your clean up crew nplan a busy day!
Make up 20 to 25 gallons of salt water and let it sit. Remove all of your rock to the garage. Use a big rubber made tub and put 5 or 6 gallons of water out of tank into it.
Scrub ruck piece by piece to remove growth. When the water in the tub is bad get more from your tank. By the time you are done your tank's water level will be down so it will be easier to replace and arrange the rock. Then fill with new salt water. Give it a day or so then add your janitorial crew and a lawnmower or sailfin blenny. Let them all scavenge
the rock and they should keep up with it. The algae needed food to grow so you need to figure out where it came from. Was it phospates in your water supply, or have you been feeding the tank? Whatever the case eliminate it! Good luck, I have battled this before
and it has now been almost 2 months with a re occurance!!!! Wax32 on this site set me straight I hope this helps you