I need to remove a URL from Health -> "Fetch as Google". is it possible?
You can't remove an url from the list. Like paulmorriss said, there is no harm in simply trying again if you made a typo error in the URL. You get your 500 fetches reset once a week.
If you want to remove an url from the index then when you are in Webmaster Tools go to Optimization > Remove URLs and follow the instructions.