Mercury Outboard 50 Hp ELPTO Prop Slippage??

I have a 2008 Mercury Outboard 50 Hp ELPTO 2 Stroke on a Triton VT17 Welded Aluminum boat. While traveling along in water that had a fair amount of floating debris at low speed, the boat stopped making forward progress but the Rpm didn't change. I heard a slight "bump" just before this occurred but nothing that sounded as though anything actually hit the prop. Even so, it seemed like the prop must have spun on the shaft. I shut the motor off and inspected only to find a small tree branch just floating at the rear of the boat by the prop. After moving it away, I started the motor and started moving until I increased Rpm. The Rpm climbed as it should but the boat wasn't moving.
Once I got back home, with the engine not running, I put the engine in forward gear and tried to turn the prop by hand. Turning it counter clockwise, there is a fair amount of resistance but the motor turns over. Turning the prop clockwise, the prop turns easily and I just hear a "clicking" sound in the lower half of the motor. I then put the engine in the reverse gear and the prop is difficult to turn in either direction.
I removed the prop and took it to the Dealer where I bought the boat. They say it doesn't appear that the prop has spun and that I should either bring the boat and motor in or just bring in the lower unit.
Has anyone had a similar issue and / or can anyone help to trouble shoot this problem?

Your observation sound correct in that in reverse the prop should be "locked" in both directions. In forward it should lock in one direction and click in the other (slipping the clutch dog - totally normal - designed that way so if you "beach" the prop you don't eat the gears).

To check if the prop is "spun" you can place a mark on the hub and a corresponding mark on the blade portion and try it in the water - if the marks become "un-aligned" then the prop is spinning on the hub and it needs service/replacement.

Alternately, if you have a flushing device you can fire up the motor and just shift it into forward - if the prop doesn't turn at all - hub or anything, then you have probably chewed something up.

From your check it doesn't sound like you have killed a clutch dog, pinion gear, forward gear etc, so I would try the trick with the marks on the prop/hub and if the marks move, find a different shop...

Thank you very much! I'll try both, first with the hose / flushing device then, if positive results, I'll mark the hub and prop and get in the water. I'll let you know!
Jeff