From my TCM notes:
Observation of Tongue coating
Tongue coating is related to stomach Qi steaming upward
Thin whitish - stays unchanged << normal
Light coloured white coating - interior cold syndrome
White powder pills - external path / extreme toxic heat
Add to this that all this can chenge within a few minutes. Drink a cup of coffee and your stomach qi is dead for hours and the coating gets thicker or even turns yellow.
So the answer is possibly, parasites
do cause internal cold but only in early stages.