Hi Theo,
It's probably exactly what the error tells you:
Mendix still compiles using Java 6, while compiling something in Eclipse uses Java 7. Since it's probably due to an upgrade of Mendix versions, you should be able to edit which JRE Mendix uses in the settings somewhere?
Take a look in Mendix at Edit > Preferences and the General tab, there's a bit about JDK which is probably still ' old '