Originally Posted by Epoch
That green thing is just a rubber insulator to keep them safe. There's no significant harm that would come to the car if they were just dangling, but I'd plug it back in just to keep them safe from lazy passengers...
Just tuck it back under the dash and forget about it.
As an aside, you can short this connection to read the CEL codes. Pretty handy trick. There is tons of information on this on the internet and this board.