The film on the foot pads are likely kapton, from Wikipedia:
Aluminized kapton, with foil thickness of 50 and 125 µm, was used e.g.
on the Apollo Lunar Module. The polyimide gives the foils their
distinctive amber-gold color. Space blankets are made by
vacuum-depositing a very precise amount of pure aluminum vapor onto a
very thin, durable film substrate.
I suggest that the exhaust from the RCS does not "dissipate" so much as expand. The particles contained within the exhaust would not necessarily be visible and could be deflected by portions of the structure, sufficiently to impact the kapton film on the foot pads.
The deflection of the foil is consistent with small-force thrust movements shown in the video. Considering the extremely thin material, a sneeze in our atmosphere would shake the film violently, while a tiny bit of deflected exhaust can explain the flutter in the foot pads.
I'd expect there may be a more technical explanation available, perhaps even thrust vector diagrams performed during the design of the LEM, showing that the impact on the pads was within acceptable limits.