I've had before, can get out of control very quickly.
As stated by
@AdInfinitum if only a couple spots, take them out immediately, cut them it off the rock, clean and put back.
IF already starting to spread,
@Nick James has it right about the Mag level helping to destroy it.
Don't let it get out of control. Mine was so thick at the time it looked like half the live rock had feathers on it.
Best of luck