It doesn't appear that you are listening to what is being said here... If you do much DYI work on your rig then you wouldn't care about that little bit of brown on the white plastic. Look at the rest of that pristine white plastic...it looks brand new! Dirty fingers probably made the stain and it doesn't necessarily come of easily...or at all sometimes.

Are you planning on putting the MAF on show? If it works, it is good to go. I would move on to something else...