Is there a way to replace the "green icon"?

Is there a way to just replace the "green" thing that show up when an icon is missing? It would just be nice if I could replace it with a plain black square and then not have to worry about the space Disabled buttons take up for non-cooldown things.

No... You can only make a disabled version of the icon that is a black square.

Actually,

I'll let you in on a little trick I do that means I never have to import DIS buttons.

No matter what directory a button is in, the game will always look in ReplaceableTextures\CommandButtonsDisabled\ for the DIS button to match it.

So, if you have a custom icon of an orange mammoth, you can name it the same as something like the button for the Magnataur Warrior, and put it in a different directory. Even the default war3imported\ will work. The disabled icon for it will be the real magnataur icon's disabled version.

If you don't like that, make a black icon (which would be very small filesize, btw), and import it as something like ReplaceableTextures\CommandButtonsDisabled\DISBTNAll.blp

Then name all your other imported icons Yadayada\BTNAll.blp using a different custom path for each one. They'll all point to your black icon as their DIS button.

Maybe that was not entirely clear. Do you get it?

Got it
That should save a good chunk of space thanks.