The following red x suggests something is wrong.

Meanwhile
- Project contains no errors and compiles fine
- Project contains no path conflicts
- Project includes all files it needs to know about
- All tests pass when ran
- Code works when executed
What would cause the red x and how can i remove it?
Try looking in the problems view for a detailed breakdown of build problems and warnings:
http://help.eclipse.org/helios/index.jsp?topic=%2Forg.eclipse.platform.doc.user%2Fconcepts%2Fcprbview.htm
There could be a whole range of problems related to your project without being specific to your source code. You might not have an expected jar on your build path – for example.