The header meta is build by oxwall. If you really want to mess with it, your can try your best, by visiting ox_core folder of you installed oxwall and there you can find a file called html_document.php.
But attention - Changing the core means that you have to do the changes everytime when there's a core update for oxwall available!
I guess I don't have to say, that if you edit a file like this, you gotta know some php.
If you want to add your own static meta, so it won't be dynamic in any way, which means that meta will be the same on every page (which google doesn't like either), you can always check "ow_themes", then your theme folder and master_pages folder. In there you'll find a file called html_document.html, if it has been created by the theme creator (which should be the case!). In there you can find {$headData}. You can add extra meta below there and/or delete that part! If you delete it, then the oxwall generated meta will disappear!
Do it on your own risk and try it out, but be careful!