Thanks for your help ! What a great community ! :)
To show it on top, I removed and added it again. It works fine. It just seemed that it was not the right way to do it but ok.
By adding it again to the FlxMouseEventManager, it seems that I can click it correctly.
But I did it another way. I turned on the MouseChildren param so that I can trigger event on other objects overlapped by my gem. I did it since I need to be abble to trigger tooltips when the mouse is over buttons and I am draggind my gem.
I will look at all your other advice :) Thanks !