When it comes to melting ice and changes in sea-level, floating ice doesn't matter at all. When it melts it will take up exactly the same space as the water it displaced by floating.
The thing we have to worry about is ice that lives on land, like the Greenland ice sheet.
Oh, and about your moon-sized-rock: we already track everything bigger than a few kilometers. So, sure it might kill us, but we'll have a hundred years or so to think about it.