But, barnacles are exclusively marine and tend to live in typically erosive settings, shallow and tidal waters(for one year?!...)
That was the first point I thought about as well, looking at all the marine lifeforms that has settled around the edges (barnacles, mussles, whatsoever) seems to be there for a while, but does not live in the open ocean, but in coastal waters. There are no algae on the surfaces, which you may expect in open water. I would imagine this part was floating around near the coast for a while. Anyway, maybe 10 of the 17 months are enough to travel the distance. If the part was afloat, a single cyclon may have moved if by 200 km in a single day, so it may have made the journey much faster than the sea current.
This piece will definitely not help to locate the wreakage, but it may point to a likely scenario how the flight ended. So far it does definitely not look like a high speed impact of an intact airplane. So either it points to an inflight breakup (loss of control at altitude when the A/P was lost together with all electric power once the engines stopped) with the flaperon falling from the sky as a single piece, or a low speed ditching.
Experts will tell.