"Does taking melatonin help with brain fog/mercury toxicity in the brain?"

No. Maltonin only helps at with insomnia. its effectiveness decreses over time, and larger and larger doses tend to be needed. I used it a number of years ago, but stopped using it as it stopped being effective. At higher doses, it can have plenty of side effects. Imo melatonin is not a good supplement to use.

Detox teas such as Triple Leaf Detox Tea and Yogi Detox tea might help alleviate brain fog, however imo the most effective supplement for brain fog is molybdenum.