Coating That Forms on Teeth
Puzzle 15 of Uses for a Shoebox Pack
Question is: Coating That Forms on Teeth
and the answer is: PLAQUE
Word Meaning:
PLAQUE — (countable) Any flat, thin piece of clay, ivory, metal, etc., used for ornament, or for painting pictures upon, as a dish, plate, slab, etc., hung upon a wall; also, a smaller decoration worn by a person, such as a brooch.
Definitions from Wiktionary.