Maven build second level+ child projects using reactor option -pl

From the documentation for the -pl option it states the following:

-pl,--projects <arg>                Comma-delimited list of specified
                                    reactor projects to build instead
                                    of all projects. A project can be
                                    specified by [groupId]:artifactId
                                    or by its relative path.

The important part for you is: "or by its relative path".

So to build projectC, you simply need to refer to it by its relative path (projectB/projectC). So the command you need is:

mvn clean install -pl projectB/projectC

This is an answer from a similar question that is also relevant here.
By using artifactIds you don't have to know the structure of your project.

---

If you only use the artifactIds of the given project you have to define that correctly on command line:

help output of Maven (mvn --help)

Comma-delimited list of specified reactor projects to build of all projects. A project can be specified by [groupId]:artifactId or by its relative path

This means in your case you have to define:

mvn clean install --projects :projectC,:ProjectY

---

Notice the : that is prepended to the artifactIds to indicate you omit the groupId