I had the same problem. I noticed on mine some bare metal around the back of the gear case. Prop nut was loose too. Yamaha took care of everything and David Fritts Outdoors did the work, both are top notch IMO .

I had a prop sling a blade in 2009, prop was covered by they Powermatch warranty, I learned this from a dealer here on BBC. He said Yamaha would take care of the prop and ensuring everything was still right with the lower unit. I inquired at my local dealer about this. Parts guy told me I needed to call the 1800 number and talk to Yamaha about replacing the prop and said they would charge me labor if they checked my lower unit and found nothing wrong.

Later that day I call Yamaha as I was instructed to do and I explain my problem to the rep. First thing he asks is "have you talked to a dealer?". When I told the rep that the dealer told me to call them, he was furious (not at me, at the dealer). Apparently he called the dealer and educated them on the right way to handle my situation. He was also astonished that they were going to try and charge me their minimum labor for checking the propshaft.