This is due to the fact that Eclipse automatically defaults to JRE 6. In order for you not to get those nasty warning triangles, you will need to use the JRE 8. The following is how you might resolve it:
Right click one of your prjects > Build path > Configure build path
Make sure you are inside the Libraries submenu, from there:
Press JRE 6 > Choose remove
Now we will add JRE 8:
Add Library > JRE System Library > Workspace default JRE (jre...)
In case you want to permanentally use JRE 8 for every LIBGDX project instead of the default JRE 6. You can:
Window > Preferences > Java > Compiler > Configure Project Specific Settings > Project > Deselect "Use compliance from execution environment 'JavaSE-1.6'" > Set "Compiler compliance level" to 1.8.
That's about it.