little cross on shape - what does it mean? The shape won't change color!

The short answer is that it's a Symbol, and if you double-click it a dialog box will offer you the opportunity to edit it. When you are done, double-clicking in an empty space away from the Symbol will get you out of editing mode.

For more complete information on Symbols, go to

https://helpx.adobe.com/illustrator/using/symbols.html

Peter