Thank you for those comments
Yes, it is dust plug.
I should check this thread before I went to autoshop by bicycle today.
I drove almost 10 miles in order to figure out what it is. Lol.
The mechanic said that it is dust cover, since I have auto transmission, it seems to be a trasmission fluid leaking too. What a sad...
Anyhow, I appreciate you folks' advising.