mud flat vs piece of land

mud flat and piece of land both are nouns.

Word	Noun	Adjective	Verb	Adverb
mud flat	Yes	No	No	No
piece of land	Yes	No	No	No

As nouns, piece of land is a hypernym of mud flat; that is, piece of land is a word with a broader meaning than mud flat:

mud flat: a tract of low muddy land near an estuary; covered at high tide and exposed at low tide
piece of land: an extended area of land

Other hypernyms of mud flat include parcel, parcel of land, piece of ground, tract.

mud flat (noun)	piece of land (noun)
a tract of low muddy land near an estuary; covered at high tide and exposed at low tide	an extended area of land