Could be your coralline bleaching out. Or could be sponges. My guess is... judging by what looks like coralline in some areas on the white stuff, is bleached coralline.
Well, coralline is the purple calcareous material that eventually covers all of your rocks. So, when coralline bleaches out, for whatever reason, it turns white.
Is that stuff hard? Or can you wipe it off with a finger and it goes floating away in the current. If it does that, then it's a sponge. If it doesn't... It's bleached coralline.
Well, it does mean it's dead, but it does not mean it's gone for good. Just give it some time, and with proper water chemistry, decent lighting, and a good calcium level, it should come back. Coralline is pretty finicky for tanks just starting out. Water parameters are fluctuating, and things are settling in.
It should.
I wouldn't worry about it right now. In due time coralline covers every part of your tank. And i mean every single part.
Yes it's good. It just shows that your tank is maturing and everything is running nicely. Coralline... doesn't really do anything, besides suck up calcium.. But it does look really nice, and it gives your rocks that "straight from the ocean" look.