A lot of detailed information on the Wikipedia page. There are a ton of different ways people can come into contact with mercury, and a ton of different formulations of mercury with varying effects. For what symptoms adults may exhibit from the most common form of organic exposure, look no further than RFK Jr., who himself has stated he has mercury poisoning from eating too much tuna:

Due to the body’s inability to degrade catecholamines (e.g. adrenaline), a person with mercury poisoning may experience profuse sweating, tachycardia (persistently faster-than-normal heart beat), increased salivation, and hypertension (high blood pressure).

It also causes memory impairment and reduced IQ. RFK Jr. seems to really fit the bill here.

But for something truly terrifying, try dimethylmercury. People can get one drop on their skin, then several months later start developing symptoms, and die within a year. It even soaks right through rubber and latex gloves: en.m.wikipedia.org/wiki/Dimethylmercury

For full symptom progression, check out Minimata Disease.