Mosley is a cautious fighter, always was and always will be. When he hurts an opponent, he does not jump on him with wild abandon, he keeps his distant while trying to land big shots.
Mosley is all about defence and fast accurate counters. There was no way in hell he was going to jump on Floyd in that situation. Floyd is always in great shape, so he was able to recover fast.
Over many years of watching fights, I've yelled at plenty of fighters for letting some other fighter off the hook, by being too cautious. In this case I don't know if Mosley did the right thing or not. It would have been dangerous but the payoff would have been enormous.