While we use collective nouns for things like "a flock of birds" or "a herd of elephants," grease isn't usually thought of in terms of a group.
However, if you're looking for a descriptive phrase, you could use something like:
* A slick of grease
* A pool of grease
* A layer of grease
* A coating of grease
These phrases emphasize the specific nature of the grease and how it is present.