Is this dark muscovado sugar safe to use?
Muscovado sugar is a partially refined sugar.  Hence, it has a lot of what is essentially molasses contained in it.  The molasses can migrate, leaving whiter areas; also scraping with a spoon, e.g., can leave whiter areas.

To reconsitute, place in a jar with a piece of bread or covered with a damp cloth (what I do).  Within a few hours, it should be nice and soft as sugar is quite hygroscopic.  If you use bread, it (the bread) will probably be hard.