You might be specifying the username as a parameter in your suite xml. When you run as suite, that particular xml is used. But when you select a single test and run it, testng creates a custom testng file (with bare minimums - no parameters, no listeners - you can take a look at the file on the path that you mention).
You need to specify your xml as a template xml in eclipse if that is a standard xml that you use. Go to project properties -> testng -> Set the xml as a template xml.