Buggus Unknownus

This is me and my shadow.

I am green with a brown bit by my tail.  I have a strange white ball think on my undercarriage. I am hard on top.  I have big brown eyes.  I cling on tight to white cars even when they are being driven at excessive speed so their owner can get to the gym for a swim in the open air pool while the sun is still shining,  I am just over half an inch long.  I was viewed with distaste by owner of said white car.

What am I?