Anyone knows what is this green rubber head beside GLK350 4matic engine?
 

Thanks for any clue...

I opened the hood and look for engine oil dipstick but instead I found this...

What is this thing?

Attachment 274474

Rotate the image 90 degrees clockwise will be the view standing facing hood

engine oil dipstick

Why didn't you pull it out to check?

If it's at the back of the engine slightly to the left it's the dipstick.
It is nice to have one with so many missing and left out these days (you're not competent to check your own oil, don'tcha know)

It is at the right and the head tip is not a circle that shown in some pictures online.

I tried to pull but it did not come out so I better check before applying force to pull it.

Sorry I did not service my own car before and this is the first owned car (previously only leased one)
Try to be extremely careful not to make stupid errors ... especially being a complete rookie...

Pull on the green knob. A good tug should easily make it looser. Pull straight up afterwards, and you will see the engine dipstick be removed.