Brief Answer:
Most probably a fungal infection, please see the detailed answer below

Detailed Answer:
Hello,

I can understand your concern. The brown layer on the tongue is most probably due to overgrowth of bacteria or fungus, mostly due to antibiotic use. Antibiotics kill some bacteria while others overgrow. This may also include fungi.
Another cause for bacterial/fungal overgrowth can be poor denture hygiene.

You may use half strength hydrogen peroxide as mouthwash and sprinkle nystatin on the dentures 2-3 times per day. You may also use an antifungal mouthwash.

If this does not give any results, you may need to take stronger antifungals, such as Fluconazole, Ketoconazole, Clotrimazole or Nystatin either as tablets or an ointment on the tongue.

Meanwhile, I'd recommend you also brush your tongue when brushing your teeth followed by antiseptic mouthwash rinse.

I hope this helps.
Let me know if I can assist you further.

Take care

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