Is geese a plural possessive noun?

No, the noun geese is the plural form of the singular noun goose.

The plural possessive noun is geese's.

A possessive noun indicates that something in the sentence belongs to that noun.
A noun that does not end with an s forms its possessive by adding an apostrophe s ('s) to the end of the Word.
A plural noun that does end with s forms its possessive by just adding an apostrophe (') to the end of the Word.

Example sentences:

A lone goose glided on the pond. (singular)

The boy stuck a goose's feather in his hat. (singular possessive)

A flock of geese flew overhead. (plural)

The geese's formation was a classic V. (plural possessive)