When porting the Eclipse workspace to another computer or another java version there may happen an error like this:
"Java compiler level does not match the version of the installed Java project facet. Unknown Faceted Project Problem (Java Version Mismatch)"
Different solutions can be searched or applied but I would recommend you to check firstly if the JDK is properly installed and after checkcing this you need to right-click to the error and chose Quick Fix. After that there are shown 2 optional choices which one of them is to use your current JDK version. By choosing that you fix your workspace problem.