Again I'd have to try to push you to use the app on the front-end rather than the query on the back end as it's WAY easier. That query language isn't quite SQL and is very touchy. That's why we have this app instead.
There are numerous filter that you can employ to drill down to the stories you're looking for. If you really wanted to filter on Formatted ID, you could even do that! And if your filters get really crazy, you can set up filter logic if you want. Check out the filters section of the Help article here: https://help.rallydev.com/custom-grid#custom
If you still really want to use a query, try using FormattedID = "18" - you don't need the prefix.