If I'm not mistaken, the Mosca in GFII is killed by Vito in a deleted scene (I think he's the guy Vito knifes in the bedroom, and Strollo was the guy he beat with the paddle on the boat), and the Mosca in GFIII is a completely different person (for reasons already mentioned). My guess is they reused the name since the scene in GFII wasn't shown in the theatrical release.