The reason behind you are getting this message is one of your projects is compiling 1.6 classes. It wont matter if you only have a 1.5 JRE on the system, because Eclipse has its own compiler and only needs a 1.5 JRE to compile 1.6 classes.
Follow bellow steps to resolve the issue:
Check the Project Properties>Java Compiler section
https://www.eclipse.org/forums/index.php/t/1067175/