I think that simple serializer doesn’t have the aliases configured, you can use the one from the serialization module, the recommended practice is to get your preferred serializer from the service like below:
String xml = Context.getSerializationService().serialize(patient, XStreamSerializer.class);
The above would only compile if you have the serialization module jar on the classpath.
Alternatively, assuming the serialization module will exist at runtime, you can set the fully qualified class name of the serializer from the serialization module as the value of the GP named serialization.defaultSerializer and then your code would change to
String xml = Context.getSerializationService().getDefaultSerializer().serialize(patient);