It could be coraline algae, but I'm not sure from the pic. If so, thats a good thing.
I'm just learning saltwater tanks myself so I'm not the expert but at first glance thats what it looks like to me. If I'm wrong someone will point it out and educate both of us.
Why do I love fish? Because every time I stare into the little world I have created for them, every problem I have just floats away....