What is this loose object above the flap fairing?
It’s a neoprene seal which may be a little loose there at the end like that and made to flutter by the flow of the boundary layer air around it.  That’s a common part used sometimes around the edges of enclosures eg flaps, landing gear fairing doors, etc. 1) to prevent scuffing or abrading parts during operation and 2) to seal out boundary layer flow, reducing drag.  You can notify the airline but the problem is benign and will not affect scheduled operations.