Because there are some Java 8 JDKs/JREs that don't include JavaFX, it is marked as non-portable in MacOS. You don't want to use any such JDK/JRE, so fix it. This is easy to fix.
Now we have to tell Eclipse to compile the JavaFX files. The easiest way is to delete the line referring to them in the POM file, and tell Eclipse to use Maven to update the project. If you don't use Maven, you could do this manually, in the build path, but then when you do start using Maven your changes will get overwritten.
The JavaFX files should now compile.