Your gut is an ecosystem. lots of things can cause candida to proliferate. Killing the bacteria That it should be competing with is usually the most common cause. I wouldnt worry about mercury so much. Have you taken excessive Antibiotics at any time in your life? Lowering your stomach acid can allow them to grow more if they were already out of control.
I would recommend taking something with antifungal properties such as garlic to start killin the bastards off. Start with small doses so you dont get extreme die off!! The toxins from die off will mess with your adrenals even more