"Dynamic array" refers to block of memory allocated via malloc() which you just happen to use as array.

"Flexible array member" [0] is when you have a struct and its last member is an array with unspecified size.

An example:

  #include <stdio.h>
  #include <stdlib.h>
  
  struct Foo {
      int len;
      int* arr; // dynamic "array"
  };
  
  struct Bar {
      int len;
      int arr[]; // FAM
  };
  
  int main()
  {
      const int n = 12;
  
      // have to allocate myself; no guarante it will be nearby the rest of struct
      struct Foo* a = malloc(sizeof a);
      a->arr = malloc(n * sizeof *(a->arr));
  
      // array is part of the memory allocated for struct
      struct Bar* x = malloc((sizeof x) + n*(sizeof *(x->arr)));
  
      return 0;
  }

[0]: https://en.wikipedia.org/wiki/Flexible_array_member