What does a pentacle with a crescent moon mean in paganism?

A crescent moon to some pagans is representative of the goddess, depending on the direction it face it can on occasion be interpreted as Maiden or Crone (two of the three aspects of the goddess). A pentagram is also ( but not exclusively) a pagan symbol. The two combined could represent a pagan path with a strong leaning to the feminine aspects. I know two Dianic pagans who wear rings with that symbol.

This is NOT the only meaning of this symbol, personal adornment is just that "personal", if you want to know what a certain symbol or device means, the best source of information is the person wearing/displaying it.