What you can do is use some java code to execute that background proces(microflow) asynchronously. This means that you call 1 java action which will call another microflow and won't wait for the response. Here is the code to call a microflow asynchronously
But there are some things that you should keep in mind when doing this!! Normally all exceptions are handled by the XAS and are logged or shown in the client. But because the xas didn't call the microflow you have to handle all exceptions yourself.
From my point of view, you should set the 'higher level' microflow as a background process. As far as I know, there's no property in the modeler to set a sub microflow as a background process.