I had that too... a couple times it broke out in my tank and went nuts... I tried everything the LFS had until I finally listened to them - a few of this and a few of that WON'T make any difference - load your tank up with hermits and they "WILL" eat this stuff until it's completely gone.
I picked Hawaian hermits - left handed and striped black/grey. They fight MUCH less than blue legs and aren't as expensive as red legs. Completely reef safe and work like you wouldn't believe.
I put in 150 into a 125g system that was soo bad it was choking out my corals (actually lost most of my corals by the time I found the solution)... it was gone in no time at all... literally like a month or month and a half.
The second time it broke out (about 2 years apart and both times caused by old lights that 'slipped my mind') I caught it early and only added 100 and it was gone in about a month.
Ask your LFS is they will order you a bag and sell them to you at a discount if you purchase them when they arrive from their distributor - this is what I did... rather than paying $1.75 each it cost me $100 bucks for 150 hermits and around 75 for the 100 hermits.
Once you get the hermits working on your problem, you need to figure out what caused it and resolve that so it doesn't come back.
Best of luck,
John.