Languages & Phrases is what you need I think (or at least something you can use too). Just search there for Moderators and add a "translation" for what you want it to be.
I've done that but can't find it. Was wondering if it might be in a .PHP file somewhere but I dunno?
The actual text is Moderators: but when you search for that in the Phrases, you do get results but not for that actual bit. It consists of multiple quotes mainly from the ACP.
It looks like the phrase has the variable name 'moderators'. The phrase text doesn't include the ':' at the end, maybe that's why you weren't finding it.