Count Blah
Most likely what you are seeing is the staining from the amalgams. It is somewhat unknown as to what this staining contains. It may be from the silver in the amalgams, or it might be from the mercury. The amount of danger from them is also unknown.
Composite fillings do not stick to mercury fillings as far as I know. So they cant simply cover them up with the composite filling.
You need to go to a dentist to confirm that there are actual pieces of mercury there and not just staining. If they left pieces of mercury they should fix it by removing them.
Blah.