What is this loose object above the flap fairing? During my flight, I noticed a loose object above one of the flap fairings. You can see it in this video: https://imgur.com/wCPSEem.

Is it normal or should I notify the airline?

People pointed out to a similar question, but A) I'm not sure it's the same rubber; B) it's a different airplane model, C) the behavior of that rubber (if my footage shows a rubber indeed) is not the same, D) the other question asked what that rubber is, whereas my question is specifically about it being loose. I'm no aviation expert nor an engineer, but it doesn't feel right to me that it's loose.
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.