How to store a Transform[,] grid in an ArrayList?

I am new to Unity game dev and trying to expand my knowledge of data structs. I'm building a match-3 game, and I currently have a multi-dimensional ArrayList holding transform coordinates on a grid of x width and y height. I have game objects tied to these coordinates in world space, with the grid syncing the data back in game space. I am attempting to push these coordinates into the nested list (familyList) to be deleted pending 3 or more units being added to familyArrayList (seen below).

Here's the abridged code:

 private static Transform[,] grid = new Transform[width, height];
 public static ArrayList familyArrayList = new ArrayList();
 
 // Go through game grid line by line and add game objects from game space into family ArrayList
 for (int y = height - 1; y >= 0; y--)
         {
             for (int x = 0; x < width; x++)
             {
                 if (grid[x, y] != null)
                 {
                     // Add grid[x, y] to familyArrayList
                     familyArrayList.Add(grid[x, y]);
                     Debug.Log(familyArrayList[0]);

Output:

 Green Unit(Clone) (UnityEngine.Transform)

How can I get familyArrayList to properly story the grid coordinates and not the game object the grid coordinates are referencing? i.e. familyArrayList[0] == '3,4' Should I instead use a nested list? At a later point in the code, I want to clear the game object from the board in world space as well as set the grid[x,y] to null:

 // Loop through all family array and destroy game objects
 foreach (var x in familyArrayList)
 {
     Destroy(familyArrayList(x).gameObject); 
 }
 // Loop through all family array and set their values to null in grid as well
 foreach (var x in familyArrayList)
 {
     familyArrayList(x) = null;
 }

But I get errors like "Argument 1: cannot convert from 'object' to 'int'" and "Non-invocable member 'Piece.familyArrayList' cannot be used like a method" which I can imagine are explained by my above problem of not storing the grid coordinates properly and instead storing the referenced game object. I'm truly stumped on this problem and thank anybody in advance who can point me in the right direction to solving this.