According to grapefruit, "Oral yeast infection occurs when the bacteria living in your mouth get out of balance. There are several resident species in the human mouth, and one of them is the yeast Candida Albicans. This type of yeast is balanced out by other species of bacteria that compete with each other for space and nutrients.
Oral yeast infection appears as white patches on the tongue or inside of the cheeks. On the skin the lesions are bright red with small satellite pustules. These white patches can also be seen on the sides of the tongue. This white patch can be rubbed off, revealing red, inflamed tissue underneath". For more information see: What is Oral Yeast Infection?