Annoying overlay add This specific add keeps popping up as overlay at random.
It's always the same Facebook Maket thing.
I'd appreciate any hint on how to get rid of it.
First, see @Ifran Latif's comment. This is a good action for finding malicious apps. 

If it's simply an ad that you want to not see anymore, there are several ad blocker apps available that can help:

AdAway

AdBlock Plus

AdGuard