with(obj_Cell)
{
    if place_meeting(x,y,other)=true
    {
        if other.ID_MyCore!=ID_MyCore
        {
            //do action
        }
    }
}

with(obj_Cell)  //get every cell in the room to do the following:
{
    if place_meeting(x,y,other)=true  //if you are touching the cell that is runing the script...
    {
        if other.ID_MyCore!=ID_MyCore  //...and your core is not the other cell's core
        {
            //do the thing that should be done when two foreign cells touch
        }
    }
}