Clearly you have lots of offers on how to fix
It's confusing when nine people who all mean well tell you different things to do. After trying all of them consider the thread above I linked called pest algae challenge thread hydrogen peroxide. We aren't dosing the tank with it
That is not dinoflagellates you have. Run this test on a sample and holler back
https://www.reef2reef.com/threads/helpful-method-for-identifying-dinoflagellates.216508/
Your algae would not be hard to fix, but it will take work in simply lifting out the rocks for external application, because internal application is diluted to the target, hits nontargets, and in that lessens the attack you can do on the intended target. Nothing should be dumped in the water this is an algal invader
Lights out doesn't work because it's not dinos.
Don't blend methods. Try all others first, fully so you won't be tempted to blend methods or you might harm the corals by compound stress, only pick one method and run it until you are sure it works or not.
what's in that challenge thread a few posts up will fix your tank, and a single test rock is repeated constantly in that thread for specific strategic reasons.
All you have to do is clean out your reef and hit all areas external not internal with peroxide or even that algaefix they mentioned, but not as a tank dose, right on the target, a test rock will reveal if it works or not.
To get the sandbed portion of your tank clean, clean out the entire sandbed. Others will recommend a different way, consult the threads they have of tank fixes as you evaluate ways.
Alright so right now before I go to bed what do you recommend I do first