He's still green IMO. Doesn't mean he's not close to world class skill wise, just means his resume is mediocre and the level he is about to face in Mayweather is one he has not come close to till this point. "Green" is not necessarily negative, nor does it mean he's not a serious threat.
A green world champion? In boxing that is quite possible, which is one of the problems with the sport. Canelo doesn't have the experience behind him which I would want him to in order to be a considered on par with Floyd. But is a good contender and one of the best match ups out there. As of now I see Floyd taking this in a 12 round decision. But Canelo can cause problems. But Floyd is intelligent enough to avoid them. But who knows maybe some of his game plan comes off and it turns out to be really competitive.
If he is green, he better hurry up and ripen quickly. Mayweather is not going to give a damn whether or not he's green.
I'd say he's a little green. But Mayweather at 36 ain't no spring chicken either and past prime. It's a fair matchup.