"Pierre qui roule n'amasse pas mousse."
Yeah, it has nothing to do with the first name Peter although it does refer to humans in general. Literally, it can be translated as "Stone that rolls doesn't gather moss". For the (wiser) French, it means that you have to stay put somewhere for a while in order to garner wealth.
Hmmm, it's probably one of those sayings that haven't aged too well.
If you think about it, it could have another meaning, the exact opposite of the accepted one if you convince yourself that moss ("mousse") is not a good thing.
There was a "Les Simpson" episode titled "Bière qui coule amasse mousse", a little play on words, which translates to "Beer that flows collects foam/froth".