The green arrows / yellow circle / red X are great to help Alliances mark islands under attack.
How about adding one more marker that doesn’t have a colour associated with it like a BLACK QUESTION MARK. That could be used when the Alliance council is thinking about where to switch their focus next. You could, for example, mark two or three islands with a BLACK QUESTION MARK and then ask in-game which island does the team want to attack next. It would be much easier than describing each island individually.