mud flat vs piece of ground

mud flat and piece of ground both are nouns.

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

As nouns, piece of ground is a hypernym of mud flat; that is, piece of ground 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 ground: an extended area of land

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

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