Help what is this white stuff?

I'll admit I've seen this stuff before on the underside of rocks where no light/low flow is present, but I've never worried much because it's never been invasive or spread-and I saw less and less as tank matured and flow has been bumped up quite a bit. Then all of a sudden I see this on one of my GSP. It does not seem to be having any effect on the coral and is only visible when the coral is sleeping. Anybody no what it is, how I can get rid of it, or do I worry for nothing? Thanks in advance, Dustin