There are two types of greasy coating: one is called ni 膩 which I call “sticky”; the other is hua 滑 which I call “slippery”.
The sticky coating is more common than the slippery coating. Both types of coating share a common quality of being greasy. The main difference is that with the sticky coating the individual papillae can be seen, while the slippery coating is more greasy and oily so that individual papillae cannot be seen.
Also, the sticky coating is greasy but it adheres firmly to the tongue body; the slippery coating is more oily and seems to “slip” on the surface of the tongue. Another difference is that the sticky coating may be dry (an apparent contradiction but frequent) indicating Phlegm with Dryness of Phlegm-Heat. The slippery coating cannot be dry. A possible way of visualizing the difference between these two is to imagine spreading a layer of butter on a toothbrush: if we spread it thickly and do not push it down the bristles of the toothbrush will be completely covered and will not be seen.
If we spread the butter more thinly and press it down on the brush, the bristles will still look greasy but we can see them. Both sticky and slippery coating may indicate either Dampness or Phlegm: the sticky coating more frequently indicates Phlegm, while the slippery coating more frequently indicates Dampness. The tongue on the top right has a sticky coating. The tongue on the top left has a slippery coating on the sides (while the coating in the central crack is sticky.