The easiest way would be to add a For Each Green sprite:
Blue sprite - DragDrop is dragging
- For Each Green Sprite
- - Blue sprite is overlapping Green sprite > set opacity to 10
- - Else set opacity to 100
This doesn't actually test for which one last collided, if blue sprite is over one Green one, it will go to 10%, if it is over both, they both will go to 10%
If you really want only one green sprite at 10% (and the "last collided"), even if the blue one is touching both, then you would have to use instance variables to keep tract of when the green sprite collided - so you can know which one was last.