If you confirm don';t have nudi that disturb it, then you can try to use vitamin C.
Here is the forum that talk about this.
http://reefcentral.com/forums/showthread.php?s=acd430aa7a51fe59cf6acc439e8e1d8f&threadid=1252294&perpage=25&pagenumber=1
I have tried and get good result.
Thanks.