Which model Moto G? From what I've read the 3rd Generation one will get 6.0 soon, the first generation probably not at all. But I would think you will want a full ROM rather than an OTA (OTAs are generally patches designed to be applied to a particular software release, rather than being full firmware builds that overwrite everything, so if you are starting from something different then an OTA is unlikely to be the right thing for you).
Anyway, CM13 is a ROM based on Android 6.0, so if you were told that it is Android 6 that is correct. However it is a version of the CyanogenMod ROM based on 6.0, so not identical to Google's 6.0 ROM. I doubt anyone intended to mislead.
So for restoring, the first question is, did you take a nandroid (a backup made by a custom recovery) before flashing the ROM? If so, just go into recovery and restore it and you have you old ROM back. It really is a good idea to make a nandroid before flashing any ROM. Otherwise choosing another custom ROM is the next simplest. There may be people who have made nandroid backups of the stock ROM (searching XDA-developers is usually a good way to find those), in which case you could download and restore one of those. In the limit there's flashing back to stock and then rooting again, but I'd hope you don't have to do that.
Sorry I can't be more specific, but I'm not familiar with this particular device.