Fat bloom is caused when the cocoa butter separates from the other ingredients and rises to the surface of the chocolate, forming pale or irregular spots or blotches and it can simply be wiped off with a paper towel.
Sugar bloom looks similar but has a slightly gritty texture that you can feel when you touch. It is caused by damp or humid conditions that encourage moisture to build up on the surface of the chocolate. It can produce a dusty coating that cannot be wiped off but it does not affect either the flavor or the melting ability of the chocolate.