The Plaque Monster is an educational movie seen in "Arthur's Tooth."
The titular villain attempts to destroy Toothtown, until he is confronted by the Flighty Fluoride Scour Rangers. They defeated the Plaque Monster by wrapping him in flossing thread and tossed him down a pit.
There are eyes and mouth holes on the stomach of the Plaque Monster. This means it is probably a person (most likely a stunt actor or puppeteer) wearing a costume.
He resembles the PBS Kids character Barney from Barney and Friends.
In the original book of "Arthur's Tooth", the monster is named "Nasty Mr. Tooth Decay", and is not clearly obvious it's an actor in a costume.
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Book version (Nasty Mr. Tooth Decay)