I'll take a guess that what you saw was some aerodynamic sealing tape that was falling apart. It's job is to make the wing more efficient when these devices are stowed, sealing any gaps between it an its neighbour. I would appreciate being told about it so I could enter it as a deficiency in the paperwork. I wouldn't expect a quick fix unless a huge chunk was coming loose and it is quite likely the aircraft would fly for a few weeks after being reported, but it should be looked at before the next departure.
However, I disagree with SoS in so much that this was "speed tape" (AKA "instant airframe" or "airframe on a roll") as this stuff has no cloth substrate, unlike duct tape. Speed tape is basically aluminium tape with a sticky backing. It had many uses including sealing holes, wiring for doll's houses, keeping passenger's baggage in one piece, providing a barrier to protect curing sealant, etc. But when it is used externally and fails, it normally just pings off; it rarely flaps about.