Confusing errors in Unity

Hello guys, I have a major issue. Unity says that my monobehaviour classes arent matching the .cs file names which I is entirely wrong (checked it twice) Also it gives me the following error messages:

Assets/_Scripts/Mouse.cs(71,62): error CS1502: The best overloaded method match for Mouse.UnitAlreadyInCurrentlySelectedUnits(GameObject)' > has some invalid arguments > Assets/_Scripts/Mouse.cs(71,62): error > CS1503: Argument #1' cannot convert UnityEngine.GameObject' expression to > type GameObject'

Here is the code:

 ...
 if (!UnitAlreadyInCurrentlySelectedUnits(hit.collider.gameObject)) {
 ...
 // Check if an unit is already in the currently selected units arraylist
     public static bool UnitAlreadyInCurrentlySelectedUnits(GameObject Unit){
         if (CurrentlySelectedUnits.Count > 0) {
             for(int i = 0; i < CurrentlySelectedUnits.Count; i++){
                 GameObject ArrayListUnit = CurrentlySelectedUnits[i] as GameObject;
                 if(ArrayListUnit == Unit)
                     return true;
             }
                 return false;
         }
         else{
             return false;
         }
     }

Enclosed you will find the .cs file. I have absolutely no clue whats wrong, everything worked just fine a day ago. Maybe you can help me out? Thank you very much in advance.

[link text][1] [1]: /storage/temp/33158-mouse.zip