I dont think Mayweather would win this easy. If he did win i think it would be close. Mosley might be hittable and lack the ability to adjust, but he is a big puncher at 135 and along with Duran he is one of the most physically strong LW's off all time. Mayweather would not handle him on the inside thats for sure, he could not beat Castillo from in the pocket and almost lost the fight because he fought there. If he did beat Mosley he would do it from long range.
Mosley was immense at that weight, plus this was when his body punching was right up there. Dropped the very talented Golden Johnson 3 times in 5 rounds with body punches. Very quick but often got two involved leading in many hard defences. But he was so good that he is one of the few offensive fighters i would pick to beat FMJ. But to be fair he was never really tested at 135, due to a weak era
Mayweather. He'd pick the holes in Mosley's style with more accurate shots, but Shane's speed and just all-around energy level would make it close. Something like a clear 7-5 for Floyd imo.