It could be a normal microbial flora,but since u mentioned it smells and reappears very fast ,it looks like some nutritional deficiency or a gastrointestinal disorder on a wider prespective.but to rule out certain oral conditions which affect the tongue its better you consult a dental or oral surgeon.
Next Steps
get it diagnosed and follow the protocol
Health Tips
its better to rule out certain premalignant conditions associated with smoking and tobacoo chewing