I have never heard of melatonin being a chelating agent. It may correct some of the problems mercury causes and give the body more strength in doing that, relieving symptoms, but unless it has two thiol groups like other chelators, then it isn't a direct chelator.

Chelators grab on directly to metals and pull them out. I know there are nutrients that can compete with mercury for binding sites, or raise gluthatione levels and in doing so may indirectly cause some redistribution and elimination of some metals and toxins, but are not direct chelators.

I used to get many reactions with using products that could mobilise mercury by their actions, but they did not chelate it appropriately. I had to use the much stronger stuff to pull it out more effectively.